15x8 with a 4.5" backspace with 275's is basically the recipe for an A-body 8 3/4 that's been converted to BBP axles.
With a SBP 8 3/4 it's a little too much backspace, you would be better off with 4.25" or even 4" on a 15x8" with 275's. Basically 4.5" is too much, 4.25" would be great, 4" is borderline not enough. If you get the 15x8 with 4.5" of backspace and run 275's with SBP axles you'll almost certainly need a spacer.
