No charge

Still won't charge on the gauge or confirmed with a VOM?
And you have tried full-fielding it right; and then it works.
If that is correct then you have exactly three options;
1) a faulty regulator ground, or
2) wrong voltage into the regulator, or
3) bad regulator

The most common problem is #1; the regulator must be connected to the battery negative terminal. Follow the ground path and clean all connections.
After that, check the voltage going into the regulator and compare it to the battery voltage; it should be within .5 volt. If it isn't, then you gotta go hunting for the why.
If that is also ok, then, I'm assuming your '66 still has the mechanical VR. Sometimes the points stick, and sometimes I have had success with tapping the metal case, and sometimes it just keeps on doing it again. It's been decades and decades since I worked on one of those so I can't tell you how to fix that.