Later Voltage regulator on 1963 Valiant????

No need to use a later Vreg since you can get an electronic type that works with your wiring and connectors. Check rockauto. I bought for $11.

But, yes you can upgrade to the later Vreg. To do so, you need to wire +12 V (key switched )to the alternator. Don't use the IGN wire (coil+) since that is after the ballast and runs ~9 V w/ key in "run". I did so in my 1965 Dart, mounting the Vreg beside the alternator. I use HEI ignition on that engine so 12 V was nearby.

Finally the why. Early Vreg used "high-side control", so other end of field was grounded. Later Vreg used "low-side control", with other side at +12 V. The did that because electronics at the time couldn't do high-side switching. Current Mopars still use low-side field control.