On mine I have front 4" HD Tuff Country with thicker HD shackles (with 1/2" bolts), the rear stock leaf pack less 2 shorter leafs (remove the upper overload pack if you have it, but keep the straight thick overload leaf between leaf pack and perches), then 2.5" shackle flip with 6" HD shackles.
It sits perfectly level with no blocks at all (which is good at preventing springwrap) and the 2.5" shackle flip was also good for the pinion angle, just reversed the stock tapered shim and was OK.
Full complements of PU bushings and greasable bolts.
All of the above from OffRoadDesign, have to say Stephen knows those rigs very well and his suggestions were great!
Don't have a RTI number for flex, but on a forklift got the front 35" tire 4 feet in the air before lifting the rear. A+
