I did the same thing to my 67 Dart, and plan on using a 325/50R15 tire on a 10 inch wheel.

The tubs and springs have been moved, but when I measured for the wheels, I needed to run 7 1/2" of backspace. This was not the look I was going for. Besides, the rear disc set up that I have recommends no more than 3 1/2" of backspace.

This means I will be having my rear end narrowed by 4" on each side to obtain the proper backspacing.