Factory ammeter

I ran the voltage test, I found about 1.5 volt drop at the bulk head connector. After that connection the rest of the system didn’t drop much. I have about the same voltage on the inside of the bulkhead connector and at the battery when the car is running.
I ran the test with an old alternator and everything seem to settle down, the amp gauge was about 1/2 way between the middle and full charge. Then I put the original alternator back in and things went haywire again. Amp gauge was way up and the alternator belt was squealing a little, like it was really working. Output voltage went up to around 17. So I was starting to put the other alt. Back in and the connector pulled off the positive field wire and i stopped there. I’m going to solder on some new terminals.
Could this alternator that’s acting up be a 65 amp? I did notice in my wiring diagrams it shows different wiring for the 65 amp alt. This car also has a single pole ballast resistor, shouldn’t it have a dual being a ‘75?