One thing to remember: Valiants are on a 108" WB, Darts are 111", so a 2nd gen Valiant (or Barracuda) is always lighter than a Dart. I'm not sure where Dusters fit into this.
not entirely correct - my Scamp is 111" - - the difference IIRC is between the "fast back" cars - Dusters and Dart Sports being 108" and the "notch back" cars - Scamps and Darts being 111"