I know that this is not something that is done usually. but I have a good running 3.1l in a smashed 2000 gran prix. and a 1999 gran prix with a locked up 3.8l will the 3.1 bolt in without a lot of hassel? I know that I'll have to swap the computer, but what else?

I'm seeing issues with a computer swap when it comes to years like that. Probably more headache than it is to find another 3800. 3800'* do kinda grow on palm trees these days. As far as the engine fitting, I have no idea. Another question is, was it the same kind of transmission also?

i think 3.8s have the 90degree bolt patten and the 3.1s have the 60

as mike said though 3.8s run forever so there are a ton of good engines in cars with rusted out bodies in the yards