72 Front Bumper on a 73 Duster?
The pre-'73 bumpers are straight across, or flat or however you want to call it. There's no "peak" in the middle to match up with the grille. Your picture shows it really well actually
Being a '73 you don't have a shock mounted bumper, so, you could bolt up a pre-'73 bumper. The brackets are probably different on the bumper side but they're the same at the frame mounts. The problem would be the bodywork- the grille and lower filler panel.