Just depends on what you have and what you want. A 275/60/15 will fit with the stock spring locations and stock wheel wells, although a 1/2” spring offset from DoctorDiff would give you some more wiggle room and if you have to weld on perches anyway you might as well.
As for the rims and the width of the rear, figure out what rims you want and the backspacing they come with and go from there. Stock A-body 8 3/4 housing width, with BBP axle and drum brakes, usually works pretty well with a 15x8 with about a 4.5” backspace. With disk brakes or less backspace you’d need to go narrower on the rear end.
