35's will fit with some trimming.
I am running a 6" Fabtech lift and 35" BFG A/T tires. I did need to trim the front bumper/valance at a slight angle and pull the inner fender towards the bumper side a little bit to keep it form rubbing, just ran a bolt through the inner fender to a support on the bumper side. I also needed to trim the front of the side steps slightly as well.
But I am also running a 9" wide wheel with 4.5" of backspace, which pushes the wheel a little further out than what is necessary. Check with yuor lift manufacturer to see what wheels will work and what backspace is required. I have heard that a stock 20" will work with most lifts, but have no personal experience with that set up. If so, the stock 20" will put the wheel further inside the wheel well and less trimming should be necessary.
Out of curiosity, what is the gear ratio in your truck? Mine was 3.08 and with the lift and tires it was really slow off the line and had a hard time climbing hills or grades. I have since swapped to a 4.56 ratio and couldnt be happier with the performance of it. Just something to think about.