B battery, I ignition, A accessory. Anyway to the question, as Gord said, the ammeter is for measuring current. So the headlight/switch draw is after the ammeter.
To answer your generic question, A person can get positive/hot power to anything, from anywhere they want. Depends on if you want to get things fused and use the ammeter. Electrics were a little low tech back then. Lacking in fuses and fusible links. They got with the program later on. Your AD will run just fine if you follow the wiring diagram.
Some folks like me, don't like ammeters. Don't like running all the power up under dash onto one terminal. Voltmeters make more sense to me. 99% of all vehicles have voltmeters now....or lights, which are a better warning. Jerry would calI them Idiot lights, I like annunciator better.
They both have their uses. Ammeter tells you whether you're charging or discharging, voltmeter tells you how much voltage your battery is putting out. Of course, if your voltmeter reads low, obviously you're not charging, so the ammeter becomes pointless. If a voltmeter dies, the truck will still run.
