I ran 15x8's with 4" of backspacing with 31x10.50's and 33x12.50's and had it worked pretty good. This was with no lift, to fit the 31's I beat the pinch weld back and to make the tires not rub (well cut actually), and to make the 33's fit I had to cut out the firewall to make them fit, but they rubbed the panhard arm slightly, not badly though and a .5" spacer probably would have fixed that. Good luck with your truck.
