You mentioned that it turns over fine if you jump it at the starter relay. It tells me that you have the necessary voltage at the starter relay to activate the solenoid and the ground is present from the neutral safety switch. That's a valid test and should indicate the issue is in the direction that ends up at the ignition switch. According to the wiring diagram, Dark green/red from the starter relay to the bulkhead terminal (middle connector, terminal "L". Changes color to yellow inside and goes directly to the ignition switch. Seems simple enough. I don't know if it's specific to Mopars but their bulkhead terminals seem to have a lot of issues but probably just age. As mentioned several times, bulkhead terminals are always suspect. I've read lots of posts on electrical issues and I'd bet that 90% of the time it ends up being either the bulkhead connections or a bad ground.
http://www.mymopar.com/downloads/1972/72DartA.JPGhttp://www.mymopar.com/downloads/1972/72DartB.JPG
