Joe you are forgetting something very important .... The body vertical panels act as a truss.
I think I can best describe this in this manner.....
place a short 4x4 on the ground now stand a 4 x 8 sheet of plywood on edge 4 high 8 wide with the 4x4 at the center now push on the right and left top edge and bend the sheet of plywood.
any luck
I have a 40" wide x 84" long chuckwagon on a 40" x 48" HF trailer

and a 62" wide by 120" long barn on a 60" x 96" northern cargo trailer base so yes it can be done quite easily.

P.S. welcome to the sand box !