As said already, it's a bolt on operation with the right parts. Your '73 has standard bumper brackets, so you have the proper mounting locations on the frame for the earlier bumper.
You will need '70-'72 fenders (w/extensions), '70-72 grille and headlight surrounds, hood, hood latch and hood latch support panel, lower valance, bumper and bumper brackets. You don't need the doors. If you use '72 fenders your marker lights will match, if you use '70/'71 fenders you will need to do something about the mismatched markers.
I converted my '74 Duster over to a '71 Dart nose, making it a '71 Demon, uh, tribute. I used '68 markers to clean up the mismatch.
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