550 lbs. each side for a 720 is plenty especially if it is 2wd. Unless you plan on stuffing a small block Buick 215 or a Ford 302 (aluminum heads) you should have plenty of weight bearing load capabillity. My truck weighs just under 3k with me in it. Your 720 is a bit heavier but still under 3500lbs I am sure. The L18 weighs in with all accessories just under 300 - around 270-80 I believe. The G54B going in with accessories will clock a little over 300 so the weight load is pretty close.
Guys have had good luck with 15/16 inch bore on 4 disc - we are talking 1/16th of an inch difference. My application is full custom so I opted for the Wilwood and have good experiences with thier gear. The rear is actually a 4 link counting the lateral struts off the rear of the diff but its the same difference...
The front suspension is completely aftermarket per se but is designed, engineered, and constructed by BeeBani (forum member). It is a combination of home grown, pre-fab, and well known parts from a 300ZX and other vehicles.
All of the LCA work is his design and boxed into the existing frame. Pretty trick! The truck should handle and absorb the road like a modern car. Much better than kinpins and torsions which were riding on bump stops I had it so low - back jarring bumps.