Widest wheel and tire combo on 15x8 Ralley

275/60/15 will fit with the stock spring locations on a Demon as long as the backspace is correct.

A-body 8.25’s are narrower than A-body 8 3/4’s. With an 8.25” a 15x8 would probably need 4.25” or even 4” of backspace. Your best bet is to measure your car, you want to measure from the wheel mounting surface (drum face) back to the springs. A carpenters square works well for that purpose to keep all the measurements straight.