First, air shocks are a disaster. They're an excellent way to tear the shock mounts right out of the car, because the shock mounts were never intended to carry the weight of the car. If you run the pressure up on your air shocks to raise the back of the car, you take weight off the springs and move it to the shocks. Now your shock mounts are carrying the weight of the car instead of the springs. If you want to run the back of the car way up in the air, get different springs with a higher arch.
Second, all A-body rear axles are not the same width. An 8" wide rim with a 4.5" backspace works well on a Duster with an A-body 8 3/4 and BBP axles. But A-body 7.25/8.25 axles are narrower than an A-body 8 3/4 with BBP axles, so more than likely you will not want 4.5" backspace on an 8" rim. More like 4.25" or even 4" if you're going to run 275's back there. With the back end of the car way up in the air the clearance to the quarters will be less of an issue anyway.
If want to max out the tire that will fit INSIDE the wheel well, a 15x8 with a 275/60/15 will be about it with the springs in the stock location and no trimming on the quarter lip. Ideal backspace will depend on what rear axle you have. If you're going to run '70's street freak style with the back end up in the air like a 4x4, then you can run 15x10's and whatever you want, because the tires hanging outside of the quarters won't be a concern with the quarter lip 3' off the ground.
