9.5" wheel backspacing confirmation needed.

[JeffPeoples]

I agree with the previous post. A little more backspace may be better. I have 5 3/4" backspace wheels (9" wide) on my 72 GS and still have room to spare before the tires hit the frame rails. Previously, I was running some really tall 295/45/18 tires, and did have to hammer the edges of the inner fenders inward a little. These tires were 28.75" tall. My current tires are 295/35/18, around 26" tall. Plenty of room now. Height does make a difference.
I would think the quarter panel profile is a little different on a GS than a Chevelle, but the axle flange to frame rail relationship should be the same.
You can always put a spacer behind the wheel if you have a little too much backspace. If your tires rub on the outside at the quarter panel, there is no easy solution.