Optimally, you want the sand centered over the rear axle.....Only problem is, then you have space in front of and behind it, which means it can and usually will slide and shift around...
Plus it takes up more usable space of the bed that way....
I havent gotten around to doing it, but I had a friend that took a few 2x4's (2x6's would work even better), and made a box to fit the tub sand, with the front and rear ends fit across the whole width of the bed, and braced up against the front and back of the wheel wells.....That keeps everything from sliding around, and keeps the weight centered over the axle....
I usually run 3 or 4 bags, myself....depending on whether its 60 or 70 lb bags....(210-240lbs)....
I wouldnt run 500 lbs, especially with AAL's.....I left 700-800 lbs of motor and accessories sitting in the bed of my truck for a few days after I got home from a roadtrip to pick it up, and it broke my left AAL, and even after replacing it, my left spring pack sags about 2"...
