Are you talking about the front or rear bumper? You mentioned about the heavy reinforcement, they didn't come out with the shock absorbing "5 mph" bumper on the rear until 1974. I did see a 1974 Duster with shock absorbing front bumper but it was a "one off" which I never saw again. Are you sure you have a 1973 and not 1974-76?
I've taken apart one of those shock absorbing bumpers and they weigh over 100#'s each front or back. The rear didn't have the holes to mount the 1973 bumper brackets that I had so you'd have to be creative with it.
If I remember right the 1970-72 were different from the later models. The 1971-72 had the slot for the bumper jack while the 1970 didn't. That makes the 1973 a "one off" year. 1974-76 are the same. I'm talking about the rear bumper here.
