If the plan is an 8.25" rear I would stay with the 4.5" backspace. With the large tires you may need a small spacer, but I think you'll be close enough that the spacer would be less than a 1/4" anyway. That car has highly modified quarter openings, like super stock style. And there's a ton of rubber outside the quarter. If you're planning on cutting the quarters out it will work, otherwise you'll need to go substantially narrower than that.
Going with less backspace doesn't really help you, at least not once you get past about 3/8" of clearance to the springs.
So the measurements for the A-body rear ends, wheel mounting surface to wheel mounting surface (WMS), are like this
SBP 8 3/4 - 57 1/8”
8 3/4 w/ BBP axles - 57 13/16”
BBP 8.25/7.25= 56 3/4”
And I believe the SBP 7.25's are a little different as well, I don't have that measurement.
There's a list of all the 8 3/4 widths here on the forum. I posted it, although a lot of it was borrowed from the list over on MoParts. I added the 7.25/8.25 measurements, as well as the BBP A-body 8 3/4 measurements. And I've confirmed quite a few of the B body axles widths for different years, as well as a couple of the truck and C-body measurements too.
An accurate 8 3/4" rear axle width list
