What Wheel Backspacing for an F-Body 8.25 in an A-Body?

An F-body 8.25 is like 59.6" wide from wheel mount to wheel mount. That's about the same as a 65-67 B body 8 3/4 (the F body 8.25 is a little wider). There's no way in heck you'll fit 15x8's with a 4.5" backspace on there with 255's. I don't think you're going to fit those on there with any tire that would be recommended for an 8" wide rim.

Just roughly, that rear axle is about 1.75" wider than an A-body 8 3/4 with BBP axles and brakes. Which means you're going to need close to a 6" backspace on a 15x8 if you want to run 255's. And 255's may be pushing your luck anyway if you're planning on keeping the stock spring locations.

Before you buy rims, you're going to want to have that rear axle mounted in the car so you can measure the distances from the wheel mounting surface back to the springs, and from the wheel mounting surface out to the quarters. Those are the numbers you need to figure out what will fit your car. If you get an exact width on that rear axle, from where the wheels mount on each side, I can probably give you a better guess as to the kind of backspace you'll need. But it's going to be more than 4.5".