What parts can a 1973 Swinger use from other Swingers?

You're really going to have to be more specific if you want a decent answer. Some parts interchange '73-76, some will interchange '67-76, and some parts will interchange between models so they can be '73+ and well beyond. BBP rear brake parts can go all the way up to '89, but you may not have BBP brakes with a '73. And some of the '73 parts are 1 year only, like all of the window regulator and internal door parts. Or the front bumper.

If you want specifics, you'll have tell us more about your car and the parts you're looking to interchange.