can it be swapped out with a 4.3 without any major changes? or will it need new motor mounts and tranny?

The engines are completely different. The 3.1 is a 60 degree Vengine, while the 4.3 is physically larger, like a small block Chevy with two cylinders missing, and has a 90 degree angle between banks.

Basicly you would be going through the same motions as putting a V8 in.