Key is this. Cast aluminum will not tolerate any flex. Even a teensey bit of flex say 1/32" will cause metal fatiuge and ultimate failure. Boxing will not get rid of it as it will torsionally work the mounting bolts. Aluminum is fine in compression but not in tension or torsional loading. When is the last time you have seen aluminum bolts holding any important fixtures together?. If the frame flexes in the rear it transfered the load all the way to the bumper in front. No way around it on a ladder type riveted frame.

Only way to change bias is to add equal weight to the rear axle as the front which is impossible in a truck unless it has a constant load in the bed and impratical for custom suspensions.

A twin i beam frame will torsionaly load just like an eary slick one and you can watch the bed do the same trick side to side with the cab. It is all part of the design.

If any one can come up with any engineering to rebut me please feel free to post it.

Garbz