The hot rod has developed a nasty habit of waggling around between 13 and 16 volts rapidly. I'm wondering if there is belt debris in the alternator causing fluctuations or if it's simply a problem with the old mechanical style regulator.
I had pulley misalignment issues that turned a belt into dust on the alternator. The front of the alternator has more rubber on it than the starting line at the drag strip.
The alternator is a 140 amp unit off a 5.9 Magnum motor.
The regulator was replaced a few years ago.
All connections are solid as far as I can tell.

The voltmeter waggles back and forth quicker than one a second. I can hear it in the fuel pump and see it in the headlights and dash lights.

At idle, I can control it a bit with increased load by turning on the electric fans and such. At speed, it's beyond control.

Any ideas?

are you sure you still have a mechanical regulator?
The old style switched out to electronic replacements about 8 years ago. you would have to order the mechanical.

Second, where is your sensing wire coming from. I know most of your wiring isn't stock, so where did you get keyed on voltage from?

Give the alternator a wash out with O'Rielly's spray brake clean.

Good point Andy. I didn't think about the replacement regulators. I honestly don't know how many years ago it was replaced.
It used to run normally but it sat for a while. It's been waggling about since I fired it up again this year.

Stumpy, I'll give that a shot.

Dump the mechanical regulator for an electronic. Maybe the contacts are rusted or have some crud in the way of making good contact? Also, check the sense wire for bad contact as in corroded crimp or...

What does a voltage drop from the positive of the battery to the hot field wire on the back of the alternator case produce?

The battery is in the trunk. That's a long way from the alternator.
What should it produce?