I'm fairly certain the generations that interchange (physically) are 70-72 and then 73-76.

'73 thru '76 I can tell you with absolute certainty interchange without problems.

'70 and '71 have the square, flush marker light while '72 only has the "corporate" plastic marker.

Dart & Dart Sport or Demon fenders do not work. Plymouth with Plymouth, Dodge with Dodge. Unless you want to make a Plodge or a Dartmouth?