I've got a '75 Duster and I was able to get a 28/10.5 M/T ET Drag under it on an 8" rim (5.5 backspace) with no modifications. I was using a '68 B body 8 3/4 (60.125" drum to drum). I had right at a 5/8" between the tire and spring.

I have since move to a Dana that I cut to 54" (disc to disc). I now use a 10" rim (3.5" backspace) and for now the same 28/10.5 tire. I switched to an offset shackle that is not only offset but moves the rearend back an 1" which helped with the additional length of the Dana. I have much more clearance to the spring now and can probably use a 29/11.5 QTP.

If you need a rear end and rims, all that I took out is for sale.

'75 Plymouth Duster
Phase I 451 906/590/2-660 10.75/126
Phase II 451 Stage VI/590/1050 9.82/135
Phase III 383 906/Victor-Pump gas 11.30/119

"For by grace you have been saved through faith. And this is not your own doing; it is the gift of God,"