'64 wiring harness

Correct or both

Disadvantage of larger wire can also be difficulty in making good crimps. Connections are the weak points. Upsizing the 18s to 16s in most bundles should not be an issue. Headlights, and ignition upsized to 16 where it was 18 IMO would not hurt. However in general I think a headlight harness using relays is a better way to take the load off the main circuit and headlight circuit.

The A1A wire in the photo appears to show heat damage. Could have been due to a short to ground or charging a dead battery from the alternator. The latter case should also show damage on the R6 wire.