You should be able to get a 275 tire in the rear if the wheel used has the correct offset to take full advantage of all the wheel well space.

I just measured, I have 245/60/14 tires on my factory 14x6 Rally II wheels, and I have a total of 1 inch of extra room combined space on the inside and outside of the tire. Which means a 270 width tire, so figure a 275 if you roll the inside of the rear fender lip a bit.