Sorry man i havent been on the site in a while. Its going to be a tight fit with 305/55's and 20x10's with just a leveling kit. You will end up cranking quite a bit and trimming. But it can be done. I've seen a 04 sierra with 20x10's and 33x13.5 tires. Its just a matter on how much you want to crank. My wheels have a 4.5" Back spacing and a -25 mm offset.