Ordering prostars what backspacing on 9 inch?

Depends on where you want the rim. I have 9's w/ a 6.0 b/s. Took off the quads but the tire sits inside the well like it's supposed to. No rubbing issues. If you move the wheel out, say 5.5" b/s, you may get some fender rub. Plus I think cars look like crap w/ the wheel/tire sticking out of the well. JMO though.