Flickering head lights, what am I missing

This is why we completely rewire these old cars, I didnt want 52 yr old wireing in mine ..

the wiring has been upgraded, bulkhead connectors bypassed for the main power feed, dash voltage regulator swapped for a volt meter,,,,,,everything of consequence runs through relays,,, as mentioned in post #11

a question here, with a car at idle, good charge on the battery and a digital multimeter probing the battery terminals, how much of a voltage swing would you expect to see?
and the same question with the motor at say 2000 rpm?