are you do a shackle flip or just flopping the axle on top of the spring as is?
are you tubbing the bed or is it stock tubbs?
I ran 32x11.50x15 on a 10" rim with a 3.75 backset with out rubbing with a shackle flip axle flop.
the 276/60/15 and 295/50/15 on a 10" rim 4.5 backset fit with stock tubbs.
if it was tubbed to the frame with a stock truck width axle a 10" rim with 7.5 backset fit a 29x15.5x15 M/T in the fenders.
so a tall/wide 18-20 should fit a tall fatty in the fenders.
as far as tall goes..I have had 34x9.50x15 on a 8" rim with 4" backset on my stepside with a flip flop.. its up in the fender dead center with stock width truck axle.
do they measure those big rims the same as smaller rims for width/backset???