So far as prototyping, someone could take the USA -1 monster truck kit, and fab something up with it to see what works best. Styrene plastic is a lot easier to fabricate with than steel!
I suggest this kit because it's been available since the early 90s annual kits but is still a long bed. IIRC (not looking at one at the moment, not at home) it has the stock half ton components in it , besides the monster truck suspension.
I've got some experimentation going on, with the vintage kits and new ones, to make a 1/25 scale Rawhide. It'll take some fabrication and a fair bit of scratch building, but if it works out, I might be able to make limited production conversion pieces for the 400s AMT forgot to make....