alright ask your self how low you need to go be cause but taking the bump stops off the vehicle and loosening the torsion bols can get you a **** load of drop, im sitting at like a 7-6 drop with the 20s its pretty damn low, if you need to go lower than that use airbags or notch the frame and cut away the bump stop mount, and take the torsio bars out, even then you can only take so many leafs out of the rear and a 4 inch block is the biggest ive seen, and the rear is alot harder to lower than the front when your trying to ge tthan many inches out of it, i have 2 leafs left and 3 in blocks, im putting 4" blocks in in the next few weeks hopefully. so the questions is how much lower ar eyou gonna go with you current tire and wheel combo, im lucky my tires tuck into the wheel wells well, but if i cut the bumpstop mounts the tires will start rubbing the inner fender. there are pics on the site to prove these points ive made as well, im not trying to be an ass or anything just making you aware.
EDIT: just looked and saw that you had a 275/45r20 THATS HUGE, get some smaller tires and it will do alot for lowering your truck and also letting your truck go lower.