I think a 12.5 tire is generally too wide for these trucks, if you care about rubbing or if you care about the sheet metal getting blasted by rocks and other debris. Those of us who care about such things generally run a 10.5" or a 9.5" wide tire.
If you don't have a lift, 33x12.50 will rub, regardless of how wide the wheel is. If you want 33s with no lift, I suggest 33x9.50s.
3.5" backspacing is ideal, but you can get away with a little less, e.g., e.g., 3.25.
If you are running stock wheels, the 31x12.50s are definitely too wide, and the 31x11s are probably pushing it.
In any event, Thornbirds are going to be two things, bumpy and loud (think WWII era aircraft).