I think the loose battery cable is the problem.

For those of you wondering how this electrical thing works....

The battery provides power to start the car. That's it's only function.

The alternator then takes over and provides electrical power to everything. There is also enough juice left over to charge the battery back to starting power levels.

But if the cables are loose the power produced is only intermittent. Then the battery comes back into use as the electrical system pulls power from the only source remaining: The battery.

So if the alternator is not charging, or if it is charging and the power is not being distributed due to a cable not making a tight connection, the battery will slowly drain to about 7.2 volts and then everything quits because that is the bare minimum needed to keep the ignition system going.

So by all means test the alternator for output. If it checks good tighten the battery cables, clean the negative connection where it connects to the car, replace the engine grounding strap and do an on car test of the electrical system. Most auto parts shops are equipped to do such a test in their parking lot.

And don't forget: This is a 1966 electrical system we are discussing. So all of my advice here is period correct. Starting about 1974 everything started to change.