The Grey Area

Now i'm sure this is done every day....You have a desirable muscle car with a rotten frame and floors.So you get a standard model that's solid underneath and transfer all the good pieces of the muscle car to the standard model unibody.
Say it was a 340 Duster and you transfer the rally dash which carries the Vin # along with the other good parts to the standard car and the fender tag to indicate all the options.The rad support and package tray will have their numbers unchanged.
..Which car is it when it's finished?