A lot has to do with the actual tires chosen. There is a lot of variance on overall height and width between tire brands and styles. The 315/75ra6 Mtr's I am running are actually true to size. They run exactly 35" on the truck and are exactly 12.5" wide. I have seen 315/75r16s that are 34.5" tall and about 11 3/4" wide. The width is a big factor. I know the coopers run small, and all of their sub brands seem to as well, like Micky thompson, **** cepek ect.
I am running a 1" BL no wheel spacers and i still rub the fire wall after hours of cutting and pounding on it with a sledge with a 3 ft handle. There is no way that you can pound the firewall back enough to clear the MTR's with no body lift. Tubbing is the only option, and honestly i figured i would smash t and try it out then if i need to tub i will just cut out the smashed metal. You have nothing to loose
I know my friend has a taco on 35's and no >body lift and I sit at the same height he does. The body lift lets me keep the suspension down and keep a real nice ride.
EDIT im also running 5.29 gears with the 35's to make it drivable. Its perfect with a 5spd and 5.29s. maybe a little high on the freeway, but i rarely drive it in the freeway and if I do i just keep it under 65 mph. Not only do i get better mpgs but less tickets!