you can run 33"s but dont get 12.5"s, i have them and they rub even with "spacers", rims compensate, they have been a hug pain in my @SS. 10.5"s will work better, i have to so lots of cutting and hammering to get in to work and they still rub. not worth the pain
and i have a 2.5" lift with no running board and they rub