1971 duster stance adjustment

Hi I picked up a 1971 duster a month ago with the 8 3/4 rear end and 10 inch drums on all 4 corners. I want to put power disk brakes up front but I have the small bolt pattern and will need to get new rims when I do the swap. I was wondering if I could fit 275 60 r15 on the rear or would I be better off with 255 60r15 if so what would I need to do to fit them ? with a 225 70r14 in the front or is there an advantage going with a 15inch up front as well with say a 225 60r15 ? I will be getting new rims as well looking at stock rally rims 14x7 up front with 5x4.5 and 15x8 in rear with a 5x4 what off sets would you guys recommend thanks