Someone is selling me a 2001 GTP trany. Will it fit my 2001 SLE? What else, other than the trany do I have to change?

Moved to General Chat.

You're going to have a very tough time with that. The 2001 Bonneville trans is IMS (Internal Mode Switch), while the GTP trans is non-IMS. There will be just enough differences (in the internal harness, certain valve body parts, line fittings, etc.) to make it not work without alot of heartache.

Your best bet in this case would be to consider the 2001 GTP trans as a potential donor trans for certain parts (HD diffy, output shaft, extension housing, maybe certain internal parts if yours are worn or messed up), and plan to rebuild your current trans.

Apart from that, you will need a tuner or vendor PCM for the new gearing, and a passenger side axle from a 2000+ Bonneville SSEi.