Let me be sure I got this right. Engine Running after start:
Ammeter shows 1/2 way between 0 and C. 14.3 Volts at Alternator and 14.2 at Battery. 0.1 V drop for 20 amps isn't terrible. Then
Turn on Headlights and apply brakes and the ammeter swings to the C.
Is that right???
Is this is completely stock 1970?
No modifications or additions?
As far as 20 amps charging. That's not surprising if the battery was only 12.0 Volts with everything off.