I just completed the C4 - AOD swap
I just completed the C4 / AOD swap, and although not complicated, you will run into some obstacles. I personally think the swap was worth it, because now I can run my car at highway speed and have the engine loaf along at 2000 rpm or so instead of 4000 rpm. I'm running a 3.55:1 gear in the rear end.
I am an amateur at best, and I've found that if I take things slowly and deliberately, I'll do OK. Saying that, I did have some obstacles that I ran into. Most are well-documented, some are not. Here's what I've learned:
1) Driveshaft shortening. As others have pointed out, you may or may not have to have your driveshaft shortened. My advice would be to get the driveshaft from a car equipped with an AOD and remove the yoke. Use that yoke to mount up to your existing driveshaft. Measure the required driveshaft length and then see if you need to have your driveshaft shortened. Yes, you can use the C4 yoke, but take it from someone who had to have the shaft worked on twice, get the AOD yoke, measure the required length, and then go to the driveshaft shop to have whatever you need done.
2) Exhaust. I am running Tri-Y headers with dual exhausts, and I had to have the flanges on the pipes moved away from the transmission case. Both pipes were hitting the case and I had to have some work done to move them around. The AOD's case is larger than the C4's.
3) Emergency brake. Because the transmission mount is further back towards the rear of the car, the connecting point for the emergency brake cable is also closer to the rear of the car. That means the two cables going to your rear brakes will now have some slack, that may or may not be taken up when you adjust the cable.
4) Flexplate / Torque converter. Be VERY CAREFUL here, and make sure that you have the correct flexplate for your setup. I didn't, and when I mated the torque converter to the flexplate and subsequently joined the transmission to the engine block, I smashed the transmission's pump and trashed the torque converter. You need to make sure the mating surface of the flexplate is the right distance from the mating surface.
I am happy to call anyone that's contemplating the swap to discuss this. Just send me a PM with your phone number.