I have a 71 Demon that is mini tubbed using aftermarket tubs.Springs are moved into the frame rail,and I can run a 12" wide wheel with 6.5-7.0 backspace,using a 12.25x31.25 Phoenix slick.That is with the wheel lips trimmed along with leading edges of the wheel wells,so that will give you an idea.

On another subject..Ron I will be PM'ing you with some questions about the combo in your 63 Plymouth.