Congrats on your purchase. Mine has a 10 bolt rear end, original to car. If yours has 12 bolt it may not be the original. I have heard that Nova's (early 70's), have 12 bolt and chassis is the same as Firebird. If you want to change to a 12 it will fit exactly. I have not tried it myself and am only going on what I have heard.
Concerning wheels, I changed front brakes to disc and with conversion kit I had to go to 15" wheels. Bought all 4, pkg deal, from Classic Industries and kept the Rally II's, just like the original 14 Rally II's.

Tire size is 225/60R15 BF Goodrich Radial T/A

They look great. Now if I could figure out how to post a picture then I could show you.