with the right backspacing wheels you can fit 35x12.50 with a T-bar crank and minor trimming of the rear of the valence, the more back spacing the better.
Is your truck your DD ? I don't know about the tires your are talking about but my BFG MT's are crap in rain and I would not want to have them on DD. I also get a fair amount of rain, they are great in snow you get a lot of snow up there so maybe they would be good for you.
edit: also as said above 12.5 wide tires should be on at least a 8-9 inch wide wheel your stock ones are only 7 inches so you should get new wheels. check out 4wheelparts.com some black steel wheels are about $60 each.