Tire Size 68 Barracuda

Tony, I like your mustang rims idea (that what the 'M" stands for).
In my humble opinion, too much backspace is MUCH more desirable that not enough. Then a quality spacer (billet, not the cast crap) can be used to put the wheel and tire exactly where you want it.
The m rims are probably perfect for the rear with discs on a b-body rear, might need a spacer on the front. Then it will come down to tire size selection.