Stripped all the old, rotten, wood off the flat bed last night to get a better view of the *a-hem*..."creative" way a PO attached the current flatbed to my '59 International. I'm not claiming to be an expert in this department, but I'm pretty sure this isn't exactly the correct way of doing this, especially if significant weight is to be placed on the bed.
I now realize I didn't get a good picture of the difference in spacing between the truck frame rails, and the frame rails of the bed, but the flatbed frame rails are about 4-5 inches farther apart than the truck frame. It works out that the inside edge of the bed rails more or less match up to the outside of the truck frame rails. My thinking was to strip off all the existing...stuff...joining the flatbed to the truck frame, and replace them with 4 or 5 pieces of large angle iron that would go from the outside of the truck frame rails to the bottom of the bed frame rails, with gussets on both ends of the angle iron to help keep it from wanting to fold down.
This is what I'm picturing, when I say gusseted on the sides (not my photo, just something I shamelessly stole off the Internets to show what I have in mind..ignore the measurements and dowel pins off the back here) -
But I'm also not sure if I need to design something in that would allow the bed to flex independently of the truck frame, or if my idea would even properly support weight on the bed. I can't imagine ever having more than 5K pounds on the bed, but then there might be that one time that I have a need to haul something considerably heavier too (there's never been anything heavier than my own body weight on the bed thus far since I bought the truck, as the wood was so rotted that I was afraid I would fall through just walking on it).