During close inspection, the Box was fitted by a work truck acc. store so I am sure there are different size options. A cool alt. to this is maybe chopping the bed sides off at the bed deck (keeping the lower bodywork and tail light portion) and bolting some equipment boxes (aluminum if you want lighter) to the actual deck of the bed (flush with the edge) making the same sort of bed box thing in a cleaner thing (could be painted to match if you want it more bling)
after looking at it if departure was an issue (and I was going this way) I don't see many issues with doing some sweet mods to the rear. (this is however the sort of thing I do)
For weight support, maybe do the older school toy thing and go with Chevy half ton spring swap. I've wheeled some hard trails fully loaded and it was like the load wasn't there. If they didn't work for the speed/whoops, maybe do a Deaver or Alcan (aftermarket chevy lift pack) and spring under the rear.
too many ideas, I need more vehicles to chop up.
Nick