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

You have to go back past ‘97 for the Crown Vic cop wheels to be 15”, those were 15x7 with a 4.25” backspace. After that they went to 17”.

And 15x7 with a 4.25” backspace won’t work for you. That’s actually the same specs as the Mopar cop rims. I have a 68-70 B body 8 3/4 in my Duster. When I ran cop wheels on it I could only fit 225/60/15’s. Now, the 68-70 B rear is wider, but only by a 1/4” per side. The Duster quarters are a full inch wider than the quarters on your Valiant. So, still not going to work.

Any 15” with enough backspace to work with that F body rear axle is probably going to have to be custom.