Basically they did get heavier over the years - similarly equipped.

According to NHRA shipping weights, 1975 had the heaviest A-bodies. 1976 got just a little bit lighter, but that's not counting the Feather Duster and Dart Lite. Those were a few hundred lbs lighter but were 6 cyl only and had limited 1976-only production.

Door crash bars were introduced in 1972 or 1973.

1973 Sports & Dusters had regular bumper brackets front & rear.

1974 Sports and Dusters got shock absorber bumper brackets on the rear only.

1975 Sports and Dusters got the shock absorber bumper brackets added to the front also.

Seat belts added a few pounds as they got more complicated over the years and added shoulder belts.

While the late 60's strippers were pretty lite, the early '70's were close and had the advantage of bigger rear wheelwells in the Dusters, demons and Sports.

Of course, as you start replacing OEM 6 cyl. related parts with V-8 engines and heavier duty trans, rear, brakes, etc, the weight goes up.