'72 Valiant has no power

Put a turbo on it. Gobs of power.

Ok, Ok, I'm sorry, I just couldn't resist.

Here's my take on it; Theres a short somewhere, or an open, possibly at the battery-clamp, that triggered the VR to send the alt into a frenzy. Meanwhile, the FL ruptured, and the ignition died from electron starvation. You cannot trust a voltmeter at the FL. If just a tiny remnant of copper completes the circuit, a voltmeter will read it. Give it a tug. Use two hands, one on either side of the flag.
If at any time you smelled burning plastic, look to the bulkhead connector, or the back of the ignition switch.
If I'm right go look for the short, before you hit the start button.Dell provided the magic which almost guarantees a solution.
If I'm wrong, try boostnit. NO, NO, kidding. If I'm wrong, the hunt is on........
Back to you, Dell