I normally start all of my projects like this in a 3d cad software and make a full plan, multiple times. and this one I made probably 4 different plans for and didn't like any of them, so eventually I just pulled off my old utility bed and just built it as I went.
This will be more of a picture documentation because I really didn't have a full plan on how to make this. But hopefully you'll be able to get some ideas from this, and if you like my design make one similar.
Step 1: The Building Process
Let me start off by saying this, i bought this truck for $500. although I've rebuilt the engine and transmission, the chassis still has nearly 280,000 miles on it. I will be the last owner of this truck because I love it and will continue to drive it until the day it rusts apart falling to pieces where it sits.
Because of that I decided to go ahead and weld the flat bed directly to the frame. I WOULD NOT suggest this for anyone else. there are multiple bolting points on the frame that you can use with the typical pucks to bolt the bed to the frame. WELDING DIRECTLY TO THE FRAME WILL COMPROMISE THE STRUCTURAL INTEGRITY OF THE FRAME. That said, to keep a low profile and mount the bed exactly where I wanted to place it I went ahead and welded it directly to the frame. That also allowed me to create a pocket in the middle of the bed so I can create a slide out drawer for tools and a set of motorcycle ramps that i can keep in the bed.
Step 2: Decking
I used metal bed frames for the angle iron to use as the cross bracing and supports for the 2x6 tongue and groove decking. The framework that goes above the truck is going to be used to hang my light bar directly below it, and will be able to be used as a materials rack once I build a second removable one for the rear.
Seal the decking with a stain and water resistant sealer.
Step 3: Add a Load
I've thrown some pretty heavy loads on my bed, everything from motorcycles to trees to huge loads of dirt and gravel. The flat bed has held up well and has never struggled to hold anything I've put on it.