Just to be clear. (Bad pun)
Duster/Sport glass is the same shape but different from the Dart. You probably knew this or would figure it out by looking. But the doors are the same.
I have found different regulator designs in the 74 and up doors than in the 73. The glass mounts differently if I recall correctly. So if the glass is changed, that has to changed over too.
A 74 door might also have a stiffener in it for side impact welded to the outside skin. Makes the door a little heavier.