hey guys, i feel kind of stupid asking this, but did all 15x8 rallys have the same backspacing?

No. and its very confusing today with all the repro wheels.

This may not be right, but I believe Corvettes always had 4" bs. Other large GMs had 4.5" bs (which works better on Camaros)

Of course, you can get repros in any offset you want (4 - 5"?)