High volts

Sounds like your grounds are good and drops thru the ignition switch and connectors is the culprit. The Vreg is doing its job fine. It commands whatever alternator you have installed "more or less" until it senses the ~14.5 V setpoint it wants. Your problem is that the sense comes after the IGN switch and you have higher voltages elsewhere, thus a drop thru the IGN switch and/or other series components exists.

It would be better to bypass the ammeter in the engine bay, so it doesn't have to loop into the cabin and back thru the bulkhead terminals (and melt it). You can do that by running a ~8 awg cable from alternator big stud to the starter relay big stud (BATT+) or straight to BATT+. However, those w/ a 63 or 65 car have separate "buss lugs" for those w/ solid screw connectors, so melting bulkheads is not a problem.