As I said, I believe these old trucks are challenged as far as wiring, fuses, ammeter. I don't want to get into how I would rewire them.
Quoting you:
"Wire from the battery +, goes to ammeter +, ......wire from ammeter - goes to the voltage regulator B terminal. But along the way the headlight switch has tagged on to this ammeter - wire going to the regulator. Diagram shows no other power wire going to the headlight switch."
"Where does the headlight switch get it's power?"
Answer, as you have said many times, as we have said, if you want to follow the 1951 diagram, H/L switch gets it's power like the diagram says. From the ammeter. Physically, if that's what your hang up is, you can wire it right from the ammeter terminal, or any other tap-in place. They are all the same power. A terminal block, a terminal of some other component, a splice, whatever. They DO NOT show a fuse to the switch except some have a little fuse for dome and brake and they have the automatic thermal overload contacts/switch. You have added a well thought out fuse panel with always hot and switched hot fuses. This is good. But the wiring diagram you say you are following does not have that, so you need to wire it per a newer wiring diagram that has a fuse panel, if you want a how-to on that area.
On the next era up (TF), they use "tell tales", a light bulb. No gauge, no ammeter, no voltmeter. They wire the pos battery to solenoid, then chain or terminal block or "chassis junction" or bounce off a component to wire directly to H/L sw, Ignition Sw, Voltage Regulator. They did add a few fuses.
Up into the 70's, still tell tale/idiot lights/annunciators . You could get gauges as an upgrade.
"Where does the headlight switch get it's power?" maybe another way to answer it is: Directly from the battery, thru the ammeter. Routing and actual to-from is a direct wire or a "tagged on" wire. Tagged on meaning: pick off the power anywhere at, or after, the ammeter downstream terminal. Whew!
