Later Voltage regulator on 1963 Valiant????

You need to tap into the same "ignition run" coming out of the bulkhead that feeds to the ballast and to the original IGN terminal of the old regulator.

Now is a good time to check out voltage drop in the ignition circuit, which is a big problem in these girls. This causes a lot of overcharging problems. The usual suspects are the bulkhead connector, the ammeter and terminals, the ignition switch and the switch connector, and in rare cases, the "welded splice" in the black ammeter wire.

One way of "fixing" that is to cut the "ignition run" wire out of the bulkhead, use the end out of the bulkhead to trigger a relay, and feed the output of the relay to the other cut end of that wire. You can pull power off the starter relay stud, through a fuse,/ breaker.