Changing out the crossover pipe, also known as the Xover (and not to be confused by the crossover coolant pipe!), is a pain in the ass.

It stradles the oil pan and goes on the left side (if you are looking from the back of the car to the front, from underneath) to the turbo where it is connected with 4 bolts.
On the right side, it goes to the exhaust headers, which are bolted to the Xover with 6 bolts, 3 for each header flange.
There's another, final, small pipe that goes from the Xover to the wastegate. If you have a one-piece crossover like me, this is a pipe that directly bolts with 4 nuts onto the 4 wastegate studs.
If you have a two-piece pipe (Lucky you! It saves a lot of time when doing the oil pan seal.), there will be two extra flanges and a few bolts that hold the two pieces together.

As you can imagine, this design is a tight fit. And once you are under the car, especially when you don't have a lift, it can be awkward to maneuver.

My old one-piece Xover pipe, after succesfully removing it from the car. If you look closely, you can see how I tried to repair it with exhaust paste (which didn't work unfortunately).

Full procedure

1. Remove the Fuel Injector Rail and injectors. I recommend following Pelican Parts' procedure
2. Remove the air box and air intake. I recommend following Pelican Parts' procedure
3. With all those systems out of the way, you are making good progress! Pat yourself on the back.
4. Now we can start with the intake manifold. You will want to remove it to get access to the Xover bolts that are connected to the turbo. I somewhat recommend following Pelican Parts' procedure, with a few caveats.

Pay close attention to remove every connection and bolt. Pelican Parts did not describe the retention bolt that sits to the left of the throttle body, right under the front edge of the intake manifold. It takes a 13mm socket and is a pain to remove. You want to buy a 13mm hinged socket wrench or even better a 'BGS Technic 220 Joint Socket Set' or similar. This will give you the clearance you need.
You will also need to remove the speed and reference sensor electrical connector mounting bracket, near the back of the manifold. After all hoses and other connections are removed, be sure to double check that you have actually removed everything, including the eight bolts that hold the intake. If you miss a bolt, the intake won't come off no matter how hard you whack it with a rubber mallet (ask me how I know). Note, when replacing the intake manifold, you'll need to buy new gaskets for it! Buy 4 of part number 94411016305 in advance.
5. With the intake manifold out of the way, you are almost ready to move on to the main event!
6. Following Clark's procedure, make your way to the turbocharger. Take note: when removing the 90 degree bend cooling water supply pipe attached to the turbo, you will lose coolant. It seems that over time, you'll lose a lot of it, so be sure to place a large catch pan under the car. To remove the 90 degree bend cooling water supply pipe, you'll need to modify a 27mm wrench. At least my wrench was too big to slide over the bolt, so I ground it down.


The two upper bolts of the turbo inlet flange are visible. In this photo the 90 degree coolant wanter supply pipe is removed.

7. You now have access to the turbo inlet flange nuts and bolts. Start by removing the two upper ones. I found it easiest to use a box wrench. If the bolts' heads are pointing towards the turbo, you can loosen them and then attach a fishing line to them using a slipknot. This ensures you won't lose the bolt in the engine.
8. Remove the third bolt on the turbo inlet flange. This is the bolt that's the lowest, but closest to the front of the car.
9. Now, first remove the O2 sensor (also known as a lambda sensor). I recommend following Pelican Parts' procedure
10. From below, continue the removal of the turbo inlet flange nuts and bolts, and remove the final bolt (there is no loose nut for this bolt as Porsche tacked on the nut). Prepare your wrists and arms because this is just lovely work. You need a 15mm box wrench (if you have the original bolts, I bought new OEM bolts and they are 17mm - go figure).
11. From below, remove the six nuts, bolts and washers from the exhaust manifold to Xover flange. The triangle shaped flanges may give you some trouble as the pipes obstruct the bolts. I recommend using a 13mm hinged socket wrench again. You'll need a second one or a 13mm socket or wrench of some kind to counter hold the bolts.
12. With the Xover turbo side and exhaust header side loosened, you are ready to remove the four nuts at the wastegate. If you have a one-piece Xover that is. If you have a two-piece pipe, you'll need to remove the nuts at the flange in between the Xover and the wastegate.
    Let me reassure you: you DO NOT need to remove the complete wastegate or exhaust to get the one-piece Xover out of your car.
    Simply remove the four nuts, and start tugging! You can give a few whacks with a rubber mallet but use downwards force only! The studs on the wastegate will probably be ready to break. Be gentle, you don't want to ruin them (and your day).
    I also recommend spraying them with WD40 or similar to ensure a smoother sliding action. Once the wastegate is a bit separated from the Xover, gently use a prying bar or screwdriver to leverage the Xover off the wastegate studs completely.
13. With the Xover off the wastegate studs, the Xover is ready to be completely removed. Give it a sideways tug and it'll come loose. You may have to separate the exhaust header side with a screwdriver or prybar. Do not scratch the flanges.
14. Get a new Xover or repair yours by welding the cracks or failed welds. It's time to put the repaired Xover back into the car. You'll need to spend money on seals as they are one-time use. Buy 3x 94411120504 and 1x 99311119500. For good measure, and to make sure everything seals up nicely, buy the Lindsey Racing copper gasket kit. This is 1x WASTEGATE INLET GASKET COPPER 944T (#9) and 2x HEADER TO X-OVER COPPER GASKET 944T.
15. You will probably also want to complete replace all your nuts and bolts as these may be rusted. Porsche replaced the Xover to turbo bolts with M10 bolts (4x n0100202 and 3x n9007300a). These have 17 mm heads and 15 mm nuts which can make it a bit more difficult to put everything together in the tight spaces. The bolts are also a thread longer than the previous version, and I suggest to cut one thread off the bolt that sits right above the bolt going in the tacked on nut on the Xover to turbo side. Leave all other bolts 100% intact.
16. Put your seals in the slotted area in the flanges on all sides. Then put your Lindsey Racing copper wastegate seal in between the flanges or on the studs of the wastegate and start putting the Xover connection on the wastegate studs. You'll need to use some force to get the Xover pipe seated fully onto the wastegate studs. Wiggle the pipe on. You can also use your rubber mallet, but try to avoid striking the Xover welds. Do not strike the wastegate studs, especially not using lateral force. You don't want to shear these off! Don't hit the pipe too hard with the mallet.
17. Put your washers and nuts on the wastegate studs, but don't fully tighten. Just a turn or two is OK for now.
18. With the Xover fully seated at the wastegate, it's time to align the pipe with the turbo side. Align it and try to put the top most bolt in (green circle in picture below). Be sure to orient the bolts as in the picture below! If you do it the other way, you won't be able to get your tools off the bolt once it is fastened. Put a nut on, but only a couple threads tight.
    You may need to use a tire iron to lever the Xover onto the turbo bolt holes so you can slide a bolt in. You can find leverage just behind the turbo, there's a metal bar/shield underneath just behind the oil level pipe.
    
19. Put in the two other bolts (red circle in picture above), using the same method. Again don't fully thread on the nuts. The final bolt will have to be done from under the car (not visible on picture above). Do not thread this bolt in, this will be done after the Xover to exhaust manifold pipe has all bolts fitted.
20. With the turbo side secured, you are ready to continue this joyous work. You will want to move to the exhaust headers, but not from below the car! From above.
21. I initially did not want to believe it but yes, you do need to loosen (not remove!) the nuts on the exhaust headers that go into the block. Doing so will give the pipes just enough slack to get them to align to the Xover. The exhaust headers are in fact two pieces.
22. Take a 13mm ratchet and wrench and loosen the 8 exhaust header nuts.
23. With the exhaust headers' nuts loosened, return to under the car and try putting the Lindsey Racing copper gaskets in between the flanges of the Xover to exhaust manifold flanges. They may slide out and frustrate you tremendously (ask me how I know). Keep at it and try to get a bolt in from above (bolt head pointing up) in one of the holes of the right Xover to exhaust header flange.
24. You'll need a few good sturdy screwdrivers or an alignment punch that are not too wide or too thick. Jam them in a Xover to exhaust header flange hole of your choosing (one that has enough space/clearance), and try to align the holes. In my case the hole on the left flange, closest to the car's nose was the most horrible. So once I got that one aligned, I knew I was in the green.
25. If you can't get a screwdriver or alignment punch in the holes because they are still partly blocked, you can use one or two C-clamps like they use in woodworking and put them on the exhaust header and Xover flanges. Tighten them, and you'll see the holes line up enough to get a punch or screwdriver in.


Try to wedge in a sturdy screwdriver or alignment punch in one of the Xover to exhaust header flange holes. Leverage them to get the other holes lined up. Put in the bolts, and continue to lever the next hole.

26. Once every bolt is in, you are ready to celebrate. I know you just high-fived yourself. I sure did. But don't overhasten! You'll want to secure those bolts with the washers and accompanying nuts. Once you have done that, take a well deserved break and a snack.
27. It's time to finish this mess. Start tightening the bolts at the turbo side completely. For these M10 bolts use 40 + 5Nm (29 +3.6 ftlb) of torque. You may need a set of crowfeet's to get the torque wrench on the bolt. I have to admit that I just put them on as tight as I could.
28. Fully tighten the exhaust header nuts again.
29. Fully tighten the six bolts and nuts at the Xover to exhaust header flanges. For these M8 bolts use 20 + 2Nm (14 +1.4 ftlb) of torque.
30. Fully tighten the four nuts at the wastegate. Be careful! These studs probably are just begging to be snapped by now. For these M8 nuts use 20 + 2Nm (14 +1.4 ftlb) of torque.
31. Using the above mentioned procedures, reinstall everything that you removed.
32. Once you are finished, be sure to refill the coolant that leaked out. You'll also want to bleed the coolant system.
33. Congratulations, you completed this job! Go out and celebrate! Take that 951 for a spin!