There are several possible causes for this.
One is voltage drop in the ground system and the hot side supply to the field (regulator) circuit, and sometimes it's simply a bad regulator
Start out by determining voltage drop
FIRST if you have breaker points ignition, you want the points closed. To determine that, put your meter on the coil NEG to ground. Turn the key to "run" engine off If you get nearly battery voltage, they are open. Bump the engine until this reading goes very low
Next, still with the key in "run" but engine off, stick one probe onto the battery POS post, the other to the regulator IGN terminal (the push on terminal). You are reading the voltage drop from the battery, through the bulkhead, the switch, and back out to the system
If this reading is more than .3V (three tenths of one volt) you have too much harness drop and should look for the cause.
Your top suspects are the bulkhead connector terminals, the ignition switch connector terminals, the switch, and in more rare cases, ammeter connections or the ammeter itself. In very rare cases, the "in harness splice" which supplies power TO the IGN switch can fail.
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Next check the drop in the ground circuit. Get the engine running at a good fast idle to simulate "medium cruise" RPM. Get the battery "up and normal." Next, stab one probe of your meter onto the battery NEG terminal, and the other directly onto the regulator mounting flange. Here again, you hope to see a very low voltage, the lower the better, and zero is perfect. Anything over .2v means you have a ground drop problem
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If these two checks show OK, make sure the battery is up and have it checked by someone who knows how to use a load tester and for dead cells
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If all the above checks out, replace the regulator, using a good name brand.
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ALSO check, remove, and clean your headlight grounds out by the radiator support. Inspect the headlight connectors, and replace them if in doubt. SERIOUSLY consider headlight relays, these never had all that much "headspace" to begin with
Inspect battery/ block/ body grounds. The battery should be grounded directly to the block. There should be at least one additional ground to the body. Factory grounds were minimal. On a V8, the same bolts for the alternator, etc which exists on the passenger head ALSO exist on the rear of the driver side head. I use a 12" no4 "starter" cable (ring terminals both ends.) Hook one end to the head with a short bolt, the other end to one of the master cylinder studs.
Clean the firewall and backside of the voltage regulator and ignition (if electronic) and use star washers.
