Now the factory determined that draw with the factory accessories and Lighting along with a 20A max output alternator to be sufficient for that size bulkhead connector and wire.
Where are you getting that from?
The factory clearly assumed the an alternator could be providing a lot more than 20 amps to the main distribution point. In fact, for short periods of time it might supply that to the battery alone.
No one is arguing about the lights. The wires from the hi/low are only 18 gage on A-bodies, and only 16 gage before then. That's why the breaker is rated 15 amps. And the number of connections in that circuit is probablematic over time. Relay system is the way to go for these cars if driven night (and not 100% restorations)