"The wiring in the headlight bucket and under the tank aren't involved with the charging system"
Well... not directly. The last problem described was overcharging and a 1.5V drop to the regulator was measured: this is a bit too much and would explain overcharging.
This voltage drop is directly proportional to current through the harness, fuses, and keyswitch up to the point the regulator wire is connected regardless whether it's a small drop from a pristine harness (including connections, fuseholder and keyswitch) or a higher drop from a degraded harness. So the rest of the system - the entire harness and the various loads - are involved with the charging system... beyond the obvious involvement of drawing power from it.
This makes it imperative that all normal loads are on when measuring voltage drop. Headlight, tail light, ignition, and the field coil. It's best done with the engine running to have a realistic ignition coil load. There will be a zero volt drop with no load current, that tells you nothing.
The fuseholders are often causing voltage drop. If any of the fuses are hot you have a problem, they should get slightly warm at normal load current but definitely not hot to the touch. The keyswitch is another trouble spot, the contacts get corroded and the springs that hold the contacts together get weak. Some contact cleaner and a few dozen cycles of the switch is all you can do without taking it apart: it's a but tricky to get apart and back together plus you have a high risk of breaking the bits that hold it together as the plastic will be quite brittle after 43 years. Aftermarket switches are available - you should add a main power relay if you use one, they tend to fail if asked to carry the full system load.
The rectifier and engine connectors should be cleaned well and have their female ends tightened a bit with pliers.
The bullet connections for the alternator coils underneath the sprocket cover are very prone to corrosion and overheating - definitely check them and resolve any problems. Corrosion there can only reduce output - they're not a factor in overcharging but they should be checked at least annually.
