73 Duster rear fitment

I have had a few 74 Dusters and currently have both the 8 1/4 and the 7 1/4. With a 8" wheel 4.25 backspacing works the best. My opinion is not to go bigger than a 255/60/15. I run 245/60/15's in the back and they fit just right. 4.5 backspace works too. any wider and you will get rub and most likely will have to do some modification