Bbody rear and Tires

Im going with a B Body REar and I want to know. What back space will i need to keep my ET radials from hitting the springs. My tires barely clear right now with a 8 1/4 and I want to use a 275 or bigger tire like max 315. And what size wheel will i need 15x10? or 15x8? I want the most traction possible.