Plan to remove flex of cab and make entire body ridge. Will be utilizing 2" square steel tubing for body frame. Original thought to allow entire new body to flex, however final use would be better suited for a ridged body.
When you're talking about the amount of force a structure with that much mass can generate, you are going to be building a bridge girder. The weight would be enormous, and there would be no benefit in doing so.
A floating second frame would be massively lighter, and more capable off road/rough terrain.
To make it rigid, you would need to design a structure capable of holding 15,000 un-evenly distributed lbs straight when hit on one corner or another with the shock load of that mass hitting a obstruction at... say... 35 mph? The springs can dampen the impact, but only to distribute it to the spring mounts on the frame. If the body goes up, the force was applied.
I'm not saying it's impossible. But such a structure made of 2" square tube would be large. Also, you would be eliminating the current cab mounting system. They float from the factory.
Treating the cab and any body structure kinda like a pick-up/Bronco with a body lift... isolated from the frame flex and only forced to hold a fraction of the vehicle weight... would seem to be the easy solution.
As for my drawings... I still have paper sketches. I'll look for the thumb-drive I had the files on. My solution was a bit different. I wanted it to be removable.