Is a Duster and Dart door swap possible?

Dart Sport will work. Regular Dodge Dart (i.e.: Swinger, Valiant, etc.) window contours are different than Duster/Demon/Dart Sport and won't fit the roof/a-pillar shape.

1973 doors have unique window mounting locations inside so if you need to replace the glass, you'll need 1973 glass. 1973 doors are also heavier than 1970 because they have side crash beams in them.