I put 275/70/18 BFG ATs on when my 100 was stock - they fit perfectly, e.g. no rub lock to lock, off-road, etc. All I did was crank the torsion bars - nudging the front end up about 1/2". But, that wasn't necessary to run those tires. Then, I lifted the truck 2.5" and bought another set of 275/70/18 BFG KO2s. I am almost certain (someone will correct me if I'm off) that they are the largest tire that will fit in the spare well. Buy 5 and do a 5-tire rotation.
Stock suspension, TBs cranked 1/2"
(friend took the picture... yeah, he's a pro)
Lifted, next set of BFGs, on the wheels you just got:
(I took the picture... with a telephone)