Need some electrical help with my ‘68 Plymouth Valiant 100

This type of problem, in addition to advice above, is generally caused by several issues

1. VR. If you still have the OEM electo-mechanical regulator, that alone can cause this
2...Poor brushes / worn/ dirty/ poor contact in the alternator
3...Other problems in the alternator such as loose / shorting stator windings.
4...As mentioned, bad connections in the ammeter circuit, including the ammeter, the bulkhead connector, the eyelet end connectors at the alternator output stud and at the ammeter, and in RARE cases, the welded splice, which is a few inches from the ammeter in the black wire. This splice feeds power to the headlights, to the ignition switch, the fuse panel "hot" buss, and depending on year / model one or two other things

To check for harness voltage drop

Turn key to "run" with engine stopped. Backprobe the IGN terminal of the VR, and stab the other probe into the top of the battery POSITIVE post. You are hoping for a very low reading, the lower the better. over .3V (3/10 of one volt) look for problems in that circuit path. This includes the ammeter circuit mentioned above, plus the ignition switch, the connector for the switch, and the bulkhead terminal feeding the IGN1 "run" voltage (dark blue) through the bulkhead connector.

GROUND DROP. Run the engine at high idle to simulate low/ med cruise RPM. Make this test first with all loads off, and again with lights, heater, etc powered. Stab one probe into the top of the battery NEG post and the other into the VR mounting flange, HARD, to penetrate rust, paint, etc. Once again you are hoping for a low reading, zero is perfect

Improving block to body ground can be a popular problem. Bolt a Ford style eyelet to eyelet starter cable from the rear of the head to the firewall. Sometimes the master cylinder mounting studs work good for that.

If wiring does not seem to be the issue, or improving it does not fix this, I would check the alternator brushes, and (I hate doing this) "throw" a new VR at the car, and most you buy now will not be mechanical, but rather a solid state replacement.

IF YOU buy a new alternator you can use a 70 or later and better yet a "square back" (rectangular heat sink) from about 73 and later, and add one new wire to convert to a 70/ later VR.

On voltage drop in the "run" circuit, one way to work around that is to add a relay in the "run" circuit to take the load off the key. Find the dark blue "run" wire coming out of the bulkhead connector and before it branches off. That wire feeds the ignition system and voltage regulator. Cut the wire where you can work with it, and use the end coming from the bulkhead to activate a Bosch style relay. Feed the relay power from the big bolt on the starter relay with about no12 wire and a fuse or breaker, 20A is fine

The switched contact connects to the engine bay end of the blue wire you cut. This will essentially give the VR a direct path to the battery. You can buy those relays from parts stores with a mountable relay socket and pigtail. Mount it with the wires facing down, so any rain in the bay will tend to drain

Pay attention to the coil polarity on the relay, as most relays of that kind have a diode across the coil. This is for spike/ surge protection when used for other purposes.