You might do what I did and find a solid used flatbed/stakebed and adapt it. I made some 2" tube crossmembers, bolted them to the frame of my '76, set the bed on them, and welded it in place. No alteration to the frame and very easy work.

That mount on ncrancheros frame with the angles over the sides (dump bed kit?) is a very strong way to go too. I have the same general setup adapting my Holmes 440 boom. Low, strong, and the angles halt lateral movement.

Check salvage yards and scrap metal places for big flatbeds you can trim (I Sawzall 'em) to size, as well as the Carolina Trader.

If you are near Sumter, Infingers is a convenient place to buy metal. I'm not affiliated with them, but they are nice folks with a good selection.