Jfet,
I read thru all 26 ppgs. and can't find a resolution to the bed suspension. Is it to be a 3 point or a diamond shaped, 4 point or other? With the length of the frame on the truck it WILL flex the bed if left in the stock position with stock attachments.
There have been some very long and technical posts over on Expo Portal regarding flat bed attachments. The simplest was the same as used on a deuce-and-a-=half army truck with tabs on the flat bed; tabs on the frame with springs between and above the and a big bolt (like 3/4") and washers holding the whole thing together so whatever wants to lift, it can. I have not tried this. In fact there is no way to make a pickup bed into a a three point system, so I just loosen the rear tie downs and let the rear float when the frame gets into a twisty mode, and I have a short bed frame. Not much frame area to twist.
I am still in awe of your build. We can all learn something here.
jefe
