I have seen this done quite a few times and IMO if you are going to do it you need to both ends (grille and taillights) or it looks incorrect. Its a lot of work and to be honest if you have a nice 73, you could probably sell it and get a 70/71 (albeit probably a much less nice car).
From my prospective the 72-74 cars are slowing coming into their own, values are increasing and we are now seeing people actually choosing them specifically (mainly due to some experience early in life). The Coda's are less of an issue as the grille is not as radical a departure from the breed as compared to the Challenger, however I really believe that the 72-74 Challengers are unique unto themselves and are just as good looking as the 70/71.
Converting a 72 up to a 70 or 71 is possible but in the end you still have what you started with and the ascetic of the front and back end is not worth the expense or time to do it, but each to his own.