It seems like I saw a lot of these when I was a kid but not much anymore. My fiancee (thanks Saabr for the proper terminology) took a photo of this while riding shotgun today on I-75 north.
One could hypothesize that the owner's Chevrolet died or gave him enough trouble to make him want to scrap the rest of the truck to make a trailer to tow behind his Ford I suppose...
Then again, if the payload capacity is the same as when it was a half ton pickup, it's lower than what a typical 5x8 trailer would be.
I'm interested in seeing some modern era truck beds being used as trailers in 20 years. Least by then they'll have like 3k payload capacities even if they're half-tons