It's been done before. You need to swap pretty much the whole nose of the car. If you can find a complete donor car then if not all that hard. Trying to find all the parts part by part could be a pain.
Mine is a '74 in which I now have a '70, '71, and '72 front end on it. You can retain your '73 fenders so your marker lights will match the rear ones, but will have to add support tabs for the latch header panel under the hood...
73 Dart....it can be done. One thing we often forget that Dart fenders are more radiused for a larfer front tire as where Duster fenders werent. You'll not be able to use a larger tire with Duster fenders w/o possibly cutting raduises larger. Dart fenders are a better choice in my book. My son derrick has a 73 Sport and the same front tire I have and didnt have to cut a darn thing like I did. JUST FYI[B/]