This whole issue goes back to ancient times when the Greeks debated whether or not a ship that had had all of its planks replaced was the same ship. We have the same debate today. Set aside legal issues for a moment....If I own Car A and Car B and I combine substantial portions of each car to produce one car, which Car has "survived?" Some might argue that the Car whose parts constitutes a majority of the complete vehicle is the survivor. Some Mopar folks might say that the Car whose cowl, dash, and core support were used is the survivor. Some might argue that both cars are "dead" and a "new" car was "born". The point is that there are different ways to operationally define the "identity" of the car....of course, all of this is ultimately superceded by legalities...and guess what, states vary in their handling of such issues!
From a legal perspective, in the state of Florida, such a vehicle is supposed to be issued a "combination" title wherein the owner woudl surrender the titles for the two original cars and a new title would be issued. Don't ask me how they would assign a VIN because I don't know. I do know that the DMV can assign a VIN to a vehicle.
From the collector's perspective, the car builder wants to claim that the Car is the actually the Car that has the greatest market value. The car buyer wants a "real" car because of its value. If the construction is done in such a way that the car can be "accepted" by the community as the "real" car (i.e. the valuable car), then both parties win, yes?
All I know is that we should all inform ourselves of state laws and abide by them. When buying a vehicle, you should research the law and ask direct, pionted questions and get the answers in writing. Anyone who intentially misrepresents the facts, whether or not their actions are illegal, is making bad Kharma for themselves.