I think this question has been asked before but I could not find the thread and I want to be clear. Will the 73 challenger bumper accept the 72 bracket without alteration?
On the rears you can weld an extension to the 72 bracket and use it with the 73 bumper. With no alteration the answer is no. You can cut out a piece on the 74 brackets to shorten them and then reattach them to the reinforcment to move the bumper in on a 74. The 70-72 bracket and bumper will go on your car without any modifications. The front brackets and bumpers are also totally different.
These are the Challenger bumper brackets.
Pic 1: 73 brackets on the top. Below 70-72.
Pic 2: 74 Challenger bumper Brackets.
Pic 3: 70-72 front bracket (shortest) with half of a 73-74 front bracket for comparison.
73 and 74 Challengers each use a different rear bumper inner reinforcement. 70-72 rear do not have a reinforcement. 70-72 front bumpers do not have the reinforcement. 73 and 74 front use the same reinforcement. The 70-72 Bumpers can be used on a 73-74 Challenger when exchanged as an assembly.