I ran 255/60/15's on stock GN wheels until I got widened GN wheels from Gbody auto recently. I had no problems with fit, or rubbing, etc..
I do have a straight frame and good body bushings though. About 1 inch clearance each side. It looked really good. I went to 275/60/15's with the new wheels, which have proper backspacing, and it fits too..
255's are pretty conservative IMO for a GN with no frame or suspension issues on the stock wheels. I know of people running 295/50's with ok results, though I wouldn't do it personally due to aspect ratio of the narrow rim and crowning of the tire.
Good luck
Billy
Montgomery Village, MD