What headlamps are in your car and what alternator? Not quite sure what your seeing but it seems similar to what I've seen.
I think the difference is in the non 1966 equipment.
When coming to a stop, the lights would dim slightly, especially with wipers on. The ammeter would often show slight discharge. Pull away and it would recharge. That was the norm and we all accepted it.
If you want to try an experiment, install a set of 6012 lamps. They had a rated low beam power draw of 40 Watts or a later H6014 H6017, or H6024 all of which I believe have a low beam draw of 35 Watts (except GE's nighthawk H6025NH).
And the other thing would to test using an alternator with a rotor that only draws around 2 amps.
I say this because in my experience the light flickering is most noticible with revised squareback alternator and HB2/H4 type lights. Both draw much more current than original - and I do remember original! LOL. We had a new '69 Belvidere - which we still had when I was old enough to pay attention to such things, and I drove '75 valiant for a long time, a long a few other A-bodies I had for shorter periods of time. The Belv was the most trouble free electric system. Only thing I recall going out was the starter. Lights were OK - not great in the wet. Didn't dim noticbly at stops as my A_bodies.
