I know with trucks, the frame itself counts as the truck, so long as it was factory-stamped with the VIN. If I ever get around to building my D-series crew cab short bed diesel, I might instead find a good '91-'93 Ram club cab long bed and do a cab and bed swap, notching the back of the cab to clear the frame rails, painting the door jambs, and then getting it inspected and the new information properly attached to the freshly-painted jamb.
There was another company building some kind of body where one had to install the original firewall into the new body, as the firewall was stamped with the VIN, but that did make it legal.
It would be interesting to find a rollover of a modern Challenger, and to take all of the goodies (suspension, airbags, computers, wiring) and put that stuff into a repro Challenger body that's been improved upon.