To recess the bumpers found on the later model Challengers (e.g. '73 & 74) that have the protruding crash bumpers (the ones that stick out.) You should will use the 70 & 71 front brackets and 70, 71 & 72 rear bumper brackets.
If it's your intention to remove the large bumperettes found on the '73 and '74 in addition to using the recessed brackets, the front is no big deal since you can remove them and simply replace where it attached with a bumper bolt. However, the rear is a little more involved and if your are needing more details I will glady share my experiences since I have just made the change on my '73 Challenger.
I agree, I like the way the later year Challengers look using the earlier year bumper brackets.