To take the best advantage of tire height to help out your rear gearing definitely go for the taller 255s, they're a good fit on a 7" wheel. Even better on an 8" wheel, but still fine on a 7" wheel.
I ran 275/60s on a 7" wheel and they were fine too, but they don't look at home as much on a 7" wheel as a 255 would.
Recommended rim width depends a little on which manufacturer's specs you look at, one of them recommended a 7.5" wheel for a 275/60 and the rest said 8" minimum. Didn't hurt one bit to use the 7" wheels with the 275s, the sky didn't fall and the car didn't try to find a ditch the first time I drove it.
