1974 Plymouth Duster Electrical Problems when hot

Murray said
You have either a bad battery or starter. Cranking voltage should never be below 10 volts. Check battery first. Charge battery fully. Load test battery, this simulates how the battery reacts to the starter running. If the voltage drops below 10v then the battery is failing under load. If the battery passes, then the starter is drawing too much.

What he said.
With the positive cable getting hot, I would lean towards a bad starter. A normal starter will pull around 90-100 amps (if I remember right). When they start going bad internally, they can pull a LOT more, and that will cause the cable to get hot. It's like this. Plug a small fan into a cheap, thin extension cord, and the extension cord will not be hot. Plug a 1500 watt space heater (12.5 amps) into that same extension cord, and the extension cord will get hot. It was not designed to carry that much of a load. I would drive it to a parts store where they do free electric system tests and see what they have to say. Hook a charger up to the battery for a while first. You have a voltmeter, so start it and see what the voltage is at the battery. With the engine running, you will have about 13.5 - 14 volts if your charging system is working. Less than 12 volts will not charge the battery.
That being said, a bad starter won't cause your car to stall. That would lead me to guess perhaps a bad (or weak) starter, battery and alternator. Without seeing in person, it is hard to tell. With what you told us, it could be all three things or any combination. It could even be a bad ground like @Clelan said. Please let us know what you find.