The logical swap would be a front subframe from a F, J or M body. Lots of 82-87 5th Avenues and Diplomats in junkyards. Measure the track width and frame rail spacing on your 55 and then go to your local pick & pull.
Welcome Pete,
Looks like some parts from my Duster will end up on your Dart.
Now if I can just get the 440 done for the truck, i can start the tear down on the Duster.