I have 325 65 r18 on the stock 18" rimes I dont know about the 20s but if you go to NITTOs web site thay have a listing of the tire sizes and what width wheel thay will fit on. I beleve the 20s are also 8" wide just look in your door jamb and as long as the tire will fit you just have to check the total clearence.
god... I could deal with the truck if they could just tone those stupid new headlights down about 100 points... rest of the truck looks great, including the tires.