Here's my Duramax 2500HD, ,,,,,,,,I put on MT/R 315/75/16's (measured height is 32.5) on the stock wheels. These tire are a bit wide for 6.5 wide rims but it works, I'l be swapping to 8" wide when I get around to it. The suspension is stock other then I turn up the torsion bars 4 1/2 turns, which is a little less then 1.5" lift. This is just enough to get the front and back level. It's not enough to screw up the CV's and I didn't notice any change in suspension stiffness, it still rides very nice. You do need to have it realigned after readjusting the torsion bars. With this tire size, you also need to trim a small 1/2" corner of body on the inside fenderwell at the rocker. This can not even be seen. The front plastic, under the front bumper also needs to be trimed,,,,,,,,I just removed the whole thing,,,,I think it looks better without it.
This is basically my tow rig and not much offroad use (yet). I have a HP D60 that will be swapped in after a little more of the warranty used up.