Not quite sure on the Dart,but I do know that 70 duster has 2 screws that hold the pigtail to the grille and 71 and up have twist in type.I think the Dart is the same?? I'm not 100% Craig.
The '72 uses 3, blade type, wire terminals in a square plastic connector. If I remember correctly, I think the '70-1 used a 2-wire, male/female, round terminal, rubberized connector.
I know 73 sockets changed to a three wire.. my 72 Dodge demon has a two pin connector while my 73 duster has a 3 wire connector... not sure best thing is to look at a wiring diagram.. good luck