74 duster rim and tire upgrade

I'm running 255s on the rear of my dart swinger with zero modifications...They fit. Your duster has much more room than my dart, as I understand you can fit 275s with stock suspension, but not sure about the backspacing.

I run 15x8 with 4.5 backspacing on the rear with 255/60r15 and 15x6 with 3.5 backspacing 215/60r15.