Also
use aligator clips and see if the voltage at the regulator drops or increases with higher rpms.
To hunt for the voltage drop.
While running, measure the voltage between regulator ground (attaching screw) and the battery neg post. This would show bad ground connections.
Then measure voltage between alternator batt stud and regulator feed (blue). This shows poor feed connection (most likely the biggest issue).
I think '65 may have ring terminal connection at the bulkhead for the alternator feed. But whatever it the connection is, find it, and measure the voltage drop to it.
Do the same the same for the ignition feed (run) coming out of the bulkhead connector. You'll need a probe to slip in there.