Click,Click,Click!

Went through exactly the same thing with my 70 Challenger, went through all the diagnostics, put in a new battery, re-did the main power wire under the dash (coming from the ignition) as that was sketchy, finally concluded that the starter was bad so I got a new one, got under the car to swap it out and discovered the main cable was super loose, tightened it up and its still working today..