i know a guy that has a 4th gen and he has the tips, muffler and the pipe that goes over the rearend.....should this fit fairly well on a 3rd gen?

 

I'm pretty sure you're going to have to modify the exhaust hangers.

 

but it should be roughly the same size?

 

For the most part

I have a LS1 cat-back on my 87 GTA & the hangers had to be changed.

LT1 cat-backs are almost a direct drop-in, except the hangers. LS1 cat backs also need to be tweaked a bit, because of the routing at the cat end.