Widest wheel and tire combo on 15x8 Ralley

Hey folks, if I was to buy some 15x8 ralleys from Summit/Jegs what is the backspacing I would need to fit the widest possible tire? It’s an 8.25 stock length rear end and never tubbed. It’s on a 72 demon. Could it fit a 275/60/15? Thanks folks been meaning to ask this for a few weeks! I know the boards will know.