I haven't noticed a problem with headlamp brightness by using the 74 front lamp harness (16 awg?). It does have a dedicated ground wire instead of the "at lamp" body screw in my 65 harness. I suspect most problems are more from corroded connectors rather than too small wires.
My 65 Newport has a bunch of rear bulbs so I used LED's for some to decrease current draw and add reliability, even if not as much light output. I expect that with future LED bulb choices any under-sized rear wiring problem will go away.
