Im thinking of buying some Z06 Wheels 18x10.5 in the back and 18x9.5 in the front. I want to put some 315s on the back what offset will I have to get for them to fit right under my 93 firebird?

 

It's going to be a tight fit, but can probably be done. The 315/30-18's I have seen have a 12.6" section width, and that's even wider than the 315/35-17 BFG drag radials (12.4") that caused a few problems on my setup (hammer inner fender liner, cut edges off jounce bumper brackets).

The stock Z06 18x10.5" wheel has a 58mm backspace, and that works out to about an 8.0" backspace..... defiinitely putting the inside edge of the tire in contact with the inner fender. You would be better off with a 7.75" backspace, which would correspond to a 50 to 52mm offset.

It's so "close" that you might want to try and find someone using the 18x10.5" Z06 wheels, and see what offset they are running. A 1/4" spacer would probably work with the 58mm stock wheel offset, but I don't particularly like running spacers in the rear, plus dealing with the issue of longer studs.