I used to sell truck equipment, including flatbeds. We even made up some custom built ones. You can build a decent bed using 3" channel for cross members, and 3 or 4 inch channel for the longitudinals. If your steel supplier has a press brake... have them shear two pieces of steel, and make sure that the sides parallel each other, and that the front and rear parallel each other. Then have the steel formed with two bends along one side of each piece of steel.
The first bend would be wide enough to slip the ends of your crossmembers into it. Leave about 1/8" extra room for tolerance, etc. The second bend comes back towards the opposite side of the sheet--forming a long, square-edged "J" along the long side of the sheet. You'll want it to be at least 1", and more if possible.
Layout the steel with the bent edges on the outside and the unbent edges facing each other. Leave a slight tapered gap to allow for welding, then weld the two pieces together.
Slip your crossmembers in and stitch weld them in. Make sure you don't weld any X-members directly above your tires so you'll have room for wheel up and down travel. You may also want to face the open side of channels to the rear of the bed to help reduce the amount of dirt, snow, etc. that accumulates there from the tires.
For stake pockets, you can use cut sections of 4" channel welded vertically along the edge of the bed, then run a length of 1/4" strap steel on the outside of the pockets for a tie down rail, and to protect the pockets.
Tack your longitudinal channels on to the X-members at the correct frame spacing. Make some triangular gussets that go between the two pieces to strengthen the bed.
You can make a headboard using the same technique, but run the side length 8 inches or so below the platform level, then weld a triangular gusset on it under the bed to strengthen it.
I realize my description may be confusing--I had a drawing I made up to show how to do it--I'll try and find it and post it.
Edited by Utah Smitty, November 26, 2013 - 06:45 PM.