1973 bumpers: significantly lighter than '74+?

It depends! But yes, the '73 bumpers are lighter, especially the rear bumper because it's the same as the '70-72 bumpers.

'74 was weird, the cars started off with shock bumpers front and rear. But by sometime in December they went away from the shock mount bumpers in the front and back to a standard, non-shock mount bumper.

The '74 non-shock mount Duster bumper weighs about 60 lbs with the brackets. The shock mount bumper would be significantly heavier (100+ lbs overall). The '73 front bumper would be the non-shock style, so, I would expect it to weigh about the same, so, 60 lbs roughly. Dart Sport would be a little different, but not a bunch.

The '74 rear bumper, with it's shock mounts and heavy reinforcing core, was over 100 lbs. I weighed all that stuff out because I sold and shipped most of it, but I can't seem to find my notes. '73's used the same rear bumpers as the 70-72 cars, and they weigh less than half that amount.