A body parts, need your help

You can't mix front clip parts between a Dart and Duster. The whole front clip/all sheetmetal together (including bumper), will bolt on, but the parts do not intermix. The '73-'76 A-bodies also had 2 different bumper mounting configurations that don't interchange. The earlier ones had conventional brackets and later ones had "shock absorber" brackets. You can't use the bumper from one on brackets from another or install the different assembly on a car that wasn't original with it without fabrication.

With fiberglass bumpers, you may be able to fabricate some lighter weight brackets to replace either style.