tire width plymouth duster

It's a stock width 8.75 out of a 70 dart.
No 1/4 panel or inner fender mods, springs in the stock location.
Axles a BBP Dr. Diff, brakes are stock 7.25 10 x 2.5"
The reason it fits so easy is 7" rims and proper back space, with 8" factory rims would probably rub.

Sorry, I should have been more clear. I was asking the OP what rear axle his car has. 275's on a Duster is a proven combination with the proper backspace, but that depends entirely on what rear axle is being used.