The thing about back-spacing is whether the person measuring it uses the outermost part of the rim or the bead area. Either way, with the sizes you're considering, not a worry. I've run 15x8 Mopar steelies with 295/50s on the rear of a '68 Road Runner and it had plenty of room everywhere at stock height. I think it was member "mickm" that said the 295 size was too tight in his '70 Road Runner in a discussion we had, but that 275s on 8s were fine.