Tough call. I'd say they are interchangable, though I've never had cause to try it. Dimensionally they appear identical. The only difference I can see (other than the signal lights you mentioned) is larger clearance recesses provided on the 73 valance to clear the larger bumper mounting brackets, though I don't expect that would be a real issue.
The 73 valance actually has extended tabs where the signal lights would (could) be, as if their deletion was an afterthought - or the 71-72 tooling was used to make it.
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