The sides are held by the front panel, three cross sils/members and the end cross bar to which the tail gate attaches. Look closely at the first pic I sent and you will see which cross members bolt to the bed sides. The corners of the bed work like buttresses and hold the sides vertical. I think it would be easy to rebuild the bed by building it upside down on a floor, it doesn't look that difficult when it's assembled.
Print out the picture and take it with you. I have done that with a lot of pictures from the forum in rebuilding the engine. I tape the picture to the firewall and do a lot of looking. Took a second look at my bed, the floor goes between the sils and the side panel angle iron, so if one was putting it together upside down, a spacer would be required until the floor was added.
Trying to get my engine started today, can't get fire past the coil, must have something wired wrong. Just had some milk and freshly baked sugar cookies, headed back out to the garage now.
Good luck
PF