275/60/15 won't fit anyway without a 1/2" spring relocation. I tried to fit a 275/40/17 on my '71 Dart and it hit the springs and the quarters with the stock spring location.
Best bet would be a 255/60/15. It'll fit on a 15x7 as well. Backspace should be about 4.25". You could probably get away with 4" of backspace, but you'd be getting close to the quarters.