[RichardJ]

18 volts generally indicates a ground problem in light of the fact that you have replaced the VOLTAGE regulator.
Measure grounds with a volt meter under operating conditions. With one volt meter test lead connected to battery -(neg) post, make all ground tests and voltage tests from that reference point.
Voltage measurements to the body of the regulator should be a couple tenths of a volt. ~ 0.2 - 0.4 Volts. Voltage measurements to the Alternator body/ engine and to the truck chassis should be the same.
Measure the voltage at the Alt output and also the Batt + plus terminal. The voltage difference between these two ~ 12 volt measurements should be only a few tenths. A higher than normal voltage drop between the Alt and the Battery, will show up as a higher reading on the dash Ammeter (Battery Gauge). You can make a direct reading from the Alt to the Batt + terminal if you choose to.

An ohm meter test for bad grounds has no value to you, because the test is being made with the meter's internal AA battery which has insufficient current for a proper test.

My '67 has the original style 37 Amp, 10 Dn, with external regulator. AC, HEI, elec pusher fan, small Sony with Blue Tooth and elec trailer brakes. It could probably use an Alternator with higher output at idle to provide brighter idle headlights, but all accessories function properly at all times and the battery is always fully charged.

The external regulated Alternator is not the source of your problems.

The old Generator voltage regulators included a Current Relay with the Voltage Regulator Relay. Alternator Regulators do not include a Current Relay. As long as the Alt regulator maintains the proper voltage, the battery will self regulate the proper charge current.

When my '67 dash Ammeter starts showing a higher than normal charge, after normal start-up, I start looking for the battery warrantee papers. From this point, I know I have a couple of weeks left on the battery.

Next electrical addition will be completion of solenoid for dual saddle tanks. Wiring is in place, but I have yet to look seriously for tank pickups and gauge sensors. This solenoid will only draw current for a spit second when energized.