Sorry for beating a dead horse about tire size

What backspace you need really depends on what rear axle you have. The A-body rear axle housings are NOT all the same size. A-body 8 3/4's are wider than the A-body 7.25's and 8.25's, and even the 8 3/4's are different depending on whether you have BBP or SBP axles or not.

With a 15x8, 4.25" backspace would only work with a 7.25/8.25 unless they car is really jacked up in the back. 275's are the widest tire you can fit with the stock spring locations and untrimmed quarters, assuming you want the tires to actually fit inside the wheel wells. If you like the '70's "street freak" style with the rear end jacked up in the air and tires hanging out like a 4x4, it doesn't matter. But if you want them inside the quarter lips you have to get the backspacing right.