Unless you strip a bunch of weight out of the Dart the Duster should be lighter than the Dart I've owned, built and race both, several different Dusters and all of them where lighter than my 1969 Dart GTS BB car stripped of the heater, radio and seats, it had a five point NHRA legal chrome moly bar in it with stock leaf springs and later the Mopar brand SS 002, 003 leaf springs It did have a all iron 440 motor with a 727 and 8 3/4 in it back then also
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
I've owned, built and race both, several different Dusters and all of them where lighter than my 1969 Dart GTS BB car stripped of the heater, radio and seats, it had a five point NHRA legal chrome moly bar in it with stock leaf springs and later the Mopar brand SS 002, 003 leaf springs 
It did have a all iron 440 motor with a 727 and 8 3/4 in it back then also 