factory aluminum trunk lid... check this out

The hood saves roughly 15 lbs and had to be crimped to the steel hood skin.
The trunk saves roughly 10 lbs.

The hoods interchange on their respective models from '73-'76 and the trunk interchanges on all Dusters, Dart Sports and Demons. The older ('70-'71?) steel trunk lids didn't have a rib down the middle.

The cars also had aluminum bumper reinforcements that were situated between the bumper itself and the "shock absorber" bumper brackets. It's a bulky, channel-style reinforcement found on the front of '75-'76 Dusters and Dart Sports and on the rear of '74-'76 Dusters and Dart Sports.
Scamps, Valiants, Swingers, etc. may have started the fronts in '74.
That saved a good bit but I haven't gotten around to weighing both versions. Those bumpers do not interchange with the earlier versions with "normal" brackets without some fabrication either way.