A '53 GMC I ued to own had 1/8" diamond plate flooring in its bed,welded down the sides and ends,and welded underneath to all the ribs,and I liked that a lot. Strong,easy to clean,and made the ol' truck ride a lot better than its '53 Chevy sister. And it still had plenty of power,so I guess it must not have weighed as much as it looked like.
My '57 GMC has some plywood patches in its bed,and some sheet metal patches,and a couple of 2X8 patches,and a few places where there'd be patches if I had anything handy to patch with...it's rough on 'em when your hoist breaks and drops engines into the bed.

My '62 one ton has mostly diamond plate decking,in a nice patchwork pattern,with various sized holes where the welding technician went temporarily blind. I see there is a little wood under the steel though. HEY-THERE'S the patches for the GMC!!!
Speed