I just replaced the bed wood in my pickup. (I know it should have a steel floor). The rotted out plywood wasn't even supported at all at the rear. I bought some pressure treated wood and made up new crossmembers and put in new boards.
Good enough for this old clunker.
(The truck too).
https://youtu.be/gCGLkEqbrjE
This was the video I was working on when I made the extra bit showing the dimensions as postred in the other thread. You can see some of the details better in this one.
Mart.