Removing the connector from the regulator creates an open circuit.
No current flows.
The voltage at both connector terminals should be within tenths of the battery.
The only drop would be from the current flowing to the ignition, which is only a couple amps.

In this situation, it probably makes more sense to measuring voltage at the alternator output stud to the regulator input with the engine running.
The closest locations to measure the votlage being sensed by the regulator while running are the alternator brush with the 'ignition' wire, and the ballast resistor connection with the same.
The 'ignition' or 'run' wires are usually dark blue or dark blue with a white tracer. But being a truck, it might be different. Later trucks used a different color, but my guess is '74 is the same as cars of those years.

If the voltage developed by the alternator increases with increasing rpm and vica versa, then it is almost certainly a regulation issue. A small variance at low rpm is normal. But going above 15 Volts is not.
If the second brush (usually connected to the green wire) gets grounded, then there is no voltage regulation. So check the brush terminal's insulator and also the insulation on the green wire to the regulator. If that's not it, then suspect the regulator.