I wouldn't be so quick to say that a hacked up 70-72 clone is more desirable than a nicely done '73 up car...
Not sure I'd hange my bumper supports off sheet metal either...Just sayin.
The bumper supports don't hang off of sheet metal on the early cars, they hang off of the rear crossmember. Which maybe is just thicker sheet metal, but that's true of the frame itself on ALL of these cars. If you use an early bumper and early brackets on a later car, you're doing almost exactly what the factory did, the only difference is how you run the two inner bolt holes that go into the open spot for the shock mount bracket. All you really have to do is weld in a piece of 1/8" sheet metal and it will be exactly the same as the 70-73 cars from the factory.
As far as a hacked up clone, again, you'd have to try to hack up a 73-76 car in the "cloning" process. Almost everything is a straight bolt on swap if you have the parts, the only things that don't bolt on are the tail panel and the rear bumper, and you already saw how easy that is. The rest is no different than changing out a busted grille, dented fender or hood. It's no different than cloning your car to an R/T, or T/A, or GT or GTS or whatever. If you use factory part, the only thing that will be different from an true early car is the number in the 6th spot on the VIN. Heck most of nice restored cars probably have more AMD panels than that anyway.