I've got an '01 'Burb with 285/75/16 Kelly Safari ATR's on some 16x8 MB Blitz wheels with +6 Backspacing (they stick out 1 1/4" over "stock" -25 bs). I've turned the stock keys up (note: not maxed!) and all I had to do was trim about 1/2" off the lower portion of the plastic front valance. I have about 1/4" of clearance front and rear in the front fender when I'm turning from lock-to-lock, don't know if I'd rub during full articulation.
FWIW, my fender heights (with the plastic stock flares) are 37" in the front, 37 1/2" in the rear. I don't know if I have any "special" springs in the rear, but I do have airbags in the rear coil springs for towing. If I inflate them from 20 lbs of pressure to 70 lbs, I do get almost another full inch of height.