The 301 was the std engine in '78 and optional in the '79 & '80. So some G-body 301 engine and frame mounts should install any '70 up Pontiac engine. The correct holes should be in the frame for the Pontiac mounts. I think the Lemans and Grand Prix bodies were on the same frame as the GA. Of course, I could be wrong--wouldn't be the 1st time.
The fuel pump and starter are on the opposite side. You can buy or make an adapter to mate the Chevy trans to a Pontiac block.
http://en.wikipedia.org/wiki/Pontiac_Grand_Am
Here's a G-body forum. Don't know if it would help or not.
https://gbodyforum.com/forums/general-discussion.3/