This is the basic circuitry.
1969 the feed to the key switch is probbaly red insulated, and the 'run' wire may go to the regulator and then to the ballast, but it works the same.

Three possibilities for high charging on the ammeter.
1. Battery is very low on charge.
* If a battery is really low on charge, given power at 14 Volts, it could draw 40 amps if the alternator can provide.
* At idle the alternator may struggle to provide that much power - hence the the drop in voltage and current.
2. High resistance in the circuits results in the regulator 'seeing' a lower voltage when more current is flowing.
* If the regulator 'sees' a low voltage, it lets more current through to the rotor. The Alternator Output might be a t 16 V while the regulator only 'see' 14 V.
* Checking each of the connectors is a good place to start. The ignition switch can also have excessive internal resistance.
3. Something else is drawing current on the battery side of the alternator.
4. The ammeter is reading wrong. It's a very simple device so unlikely but its possible. Small magnetic fueld created by the electric current deflects the needle. Easy enough to verify with a clamp meter if all else is ruled out.