They're 15x8 with 5.25" of backspace. Pretty weird, I know. But here's why. It has a b-body rear that was originally narrowed for a different pair of rear wheels and tires. I don't recall where the dimension on the axle ended up at but it's narrower than a stock B and a little wider (I believe) than a stock A.
I abandoned the black steelie idea and am going to relocate the springs, mini-tub, narrow the rear again, and go with a 325 tire. The wheels will have 3.5" of BS. Will likely sell the steelies and tires that have probably less than 25 miles on them.