Your writing about bed frame vs truck frame flex is spot on with one of my goals. When I first got the truck, I ran over a car at the junkyard just for the excitement of it. Coming out of open heart surgery and being the only male in the house, nothing was more satisfying than hearing the sound of bending metal, smashing glass, a whistling turbo, the smell of diesel exhaust, and then the sound of my tire valve stem breaking off (CTIS Run Flat mode worked perfectly to keep the truck driving for the rest of the day!). I observed that when the front tire was up on the car roof, all 4 rear wheels were still flat on the ground, because the truck frame twists to allow this for stability and to keep the tries in contact with the ground and driving forward. Hence the spring loaded bolts which hold the bed down. I don't want to limit this ability, I think this is an amazing design feature. Plus, it would be great to make the bed frame stronger if I ever do lift real weight with it.
So, I think I would be fastening the 3" or so bed lift sub-frame to the bed as mentioned, and have that hinge on the truck frame. I'd hate to be running over my daughters boyfriend's car, and tweak the dump bed sub-frame because it is affixed to the truck frame vs the bed frame.
Good ideas from everybody. Apology for my thinking out loud on this and everything else I do, but I really value the experience and lessons learned of those who have done it before.