73 Duster Mini Tubbed - How big a wheel?

Sorry I can't help you, but I mini-tubbed my 70 Dart and I can fit a 15x10 with 3.75" backspace with a 295/50/15 tire AND a 17x11 with a 4.25" backspace with a 315/35/17. I have a narrowed dana with a length of 49 5/8" flange to flange. But I think with a stock length A-body rear and a minitub, people are using a 7 inch backspace...