Should only take the tail light brackets. The later Duster's used a quarter panel brace like this, not sure if the '73 did or not. But the '72 tail light bracket should include a brace, so you'd just remove this one and install the brackets.
This is on a Demon but the '72 Duster brace is similar in that it is part of the tail light bracket
The bumper was pretty much the same for '72 and '73, no shock mounts on either so the rest of the tail should be the same, ie, it should have the corner reinforcements for the standard, non-shock bumper that are missing from the rear shock mount bumper cars (74+).
There's a lot more to replace if you swap a shock mount bumper car like your '76 to the earlier style. Not only do you need to change the tail panel, but the bumper, brackets, filler etc. Lots more work. There's a how to that covers some of that swap here Duster to Demon Tail Panel Swap