'66 Valiant Weirdness

Point gap is in the factory service manual.
if you don't have one, you may be able to download one in pdf from mymopar.com Dodge or Plymouth for the same year will be very similar.
I like a print copy too and have always bought one for the specific year I actually own.

As far as the ammeter bouncing that quick, I would suspect connections as much as suspecting the VR.
When the VR's points connect together power flows direct to through rotor's windings. That creates a spinning magnetic field. That in turn induces the electrons in the fixed windings (stator) to try to move. When the points change position, less current or no current flows through the rotor. The magnetic field starts to decay. It doesn't flip on and off - and that's the reason I don't think its the VR.

This Chrysler booklet explains that in more detail.
Alternators and Regulators (Session 228) from the Master Technician's Service Conference

The MTSC Series is a very useful resource. Here's the index at Imperal Club
Master Technician Service Conference - Chrysler's Training for Mechanics

Mymopar has most of these in pdf format if you prefer that.