Amp meters don't care what the voltage is, I would change to a 30-amp meter using an alternator but you really will only see what the battery is drawing to recharge. Once it is charged back to a normal charge state then the amp meter won't really read much of a load. It will tell you if the alternator is not working since it will show a discharge.
The model A was set up to run positive ground. Is your alternator set up for positive or negative ground? That makes a difference to the amp meter since it is polarity sensitive. I assume your alternator is a GM 10si unit. The two pin plug on them is strictly for the excitement of the field and the operation of the warning light. I would wonder how it is connected and what type of system switch is being used. The original switch (Electrolock) only controlled the ignition system and on the breaker side of the coil to boot. The screw terminal on the alternator is the one connected for charging the battery buss so it connects to the headlights, horn, and the terminal box on the firewall.
This link shows how it was originally wired.
https://www.mafca.com/downloads/Semi...ex%20Janke.pdf
Yours has been modified for a GEN warning light and you need to know how it should be wired up. We need to know system polarity and what type of ignition switch it has and how it is wired in orfer to help.