I'll be interested to hear what the experts say but my hunch is that a 255 is too meaty for a 7" wide wheel where handling is a priority. You'll end up with a lot of deflection....
Performance wise (astraight line) , you really don't want to go much past 2" wider than the rim, less for handling.
As far as being able to do it. We run 12.50's on a 6" rim on the dually's all the time.
Also, until you play with the wheel offsets/fender clearance, I think you'll run into problems at both ends.
IIRC, my 245s are really close to the leaf springs in the rear, would probably rub the fenders in the front.