Opinion on universal wiring harness

Firewall connectors are handy during assembly and diagnostics too. All of the cavities can be backprobed from eithe side, except the two power feeds on most years. Most years. Lemme say that again 'cause they are not all the same. Some years are very different than others.

On a standard '70 wiring setup the power feeds use the bulhead connector. These can see high current flow, especially with a higher output alternator feeding a low battery and other high demand situations. Chrysler had a couple strategies to provide a more robust wiring for cars equiped with 60 or 65 amp alternators alone or with rear window defrost grids. Those don't always show up in the shop manuals but you can do a search here for the drawings and even some photos.

Running the A/C at idle is one of those situations where an alternator with higher capacity at low rpm is helpful. I *think* some models/years had a A/C activated idle speed solenoid...

Gages. Factory used a reduced voltage (IVR) for these. Aftermarket usually runs at the system voltage.

Headlights flickering. Depending on the nature of the flickering at idle it could be...
voltage dropping because the alternator can't produce enough power at idle to handle everything.
alternator ripple
loose circuit connection (could even be internal to ignition switch) causing the the regulator to 'see' varying voltage.
circuit breaker weak or seeing too much current (possible wire chafing causing short).

That said, the a-body headlight circuit used 18 ga and 16 ga wires and has lots of connectors etc. It was OK with the original 6012 headlamps. But its not so OK even with the slightly higher wattage 6014 lamps that became available just a few years later. Changing the headlight circuit to power directly from the alternator using relays is a pretty good solution. This helps the headlights and also reduces load carried by the main power feeds. Then you can run any decent headlight you like without worries.