Aftermarket Head unit and speakers 73 dart custom

Best way, IMO, without having to carve up your dash would be to mount the head unit under the dash with something like this:
Under Dash Surface Mount Overhead Stereo Radio Housing Boat Marine RV Trailer | eBay
Skip door mounted speakers, you ruin your door panels and can run into clearance issues with the window mechanisms inside. Instead, either get some kick panels made for speakers like these from Year One:
A-Body 1962-76 - Dart - Duster - Valiant -- Interior / Kick Panels / Panels With Speakers / (yearone.com)
and/or buy or make an adapter to mount 2 3.5" speakers (or tweeters) in the stock dash speaker area:
[FOR SALE] - I'm back with my dual 3.5" speaker brackets | For A Bodies Only Mopar Forum
The rear package panel should accept standard 6x9" speakers.
Mount the amp in the trunk, leave the factory AM in the dash.
Some will tell you to just mount a bluetooth adapter in the glove box and play music off your phone, you can do that too.