A 275/60/15 should fit without rubbing, but the backspace has to be pretty close to perfect to keep it off the springs and quarters.
What rear axle do you have? It matters, because the 8 3/4 A body axles are actually wider than the A-body 7 1/4's.
With an A-body 8 3/4 you can usually clear 275/60/15's on a 15x7" with 4" backspace. Keep in mind though, that a 275/60/15 is not recommended for a 7" wide rim. You'd be much better off with a 15x8 and 4.5" backspace. Usually with those specs you end up tight on the springs, but a small spacer can adjust that if need be. Every one of these cars is a little different though.
