Measure the voltage drops directly.
In other words place one test lead on the power source, and the other at a connection downstream..
A 1971 factory voltage regulator will try to maintain system voltage around 14 Volts.
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Yes measure drop to the relay, but also measure drop to regulator.
The problem could be the wiring and/or it could be the alternator's capacity has been maxed out at whatever rpm you are testing at.
When the alternator can not produce sufficient
power for the demand (Power = volts x amps), system voltage drops. Then the battery becomes the highest voltage power source. But its output voltage will also drop as power is sucked out of it.
