71 Duster rear tire size

Hey, I just bought the 15x8 ralley wheels with 4 5/8 backspacing for my 71 Duster. Now im trying to find the widest possible tire that will fit w/out rubbing (hoping for 275-60-15 or 295-50-15 BFGs). Also hoping to not move the springs. Oh and my car has a built 340 out of a 1970 Cuda', 3:91 Posi, 4-speed (E-body Pistol Grip)..bla bla. All by the age of 19. Anyways Thanks a lot