The 8.25 is narrower than an A-body 8 3/4, even if the 8 3/4 hasn’t been upgraded to BBP axles.
4.5” backspace with a 15” rim and tires with a tall sidewall is going to be pretty close on the springs. I test fit a set of 17x9’s with 275/40/17’s on my Duster when it had the stock spring locations and a BBP 7.25, which is the same width as the 8.25. I used a spacer to put them at a backspace of 4.75”, they cleared but were very close to the springs.
So the 4.5” backspace will likely clear, but with the larger sidewall bulge you’re going to have with the 15” rim/tire combo it will still be very close. A thin spacer might be necessary, depending on what tires you put on those 15x9’s. On the bright side, with SS springs and that backspace you probably wouldn’t need to do much of anything to the quarters to clear a 275.
