68 Barracuda alternator question

seems to have quit charging - after a weak cranking start-up and a short drive, battery is drained below 12V and with car running my volt meter is only showing the remaining low charge left in battery where previously it would show pretty solid 14V or so. SO.. when I put a voltmeter on the output stud of alternator with engine running in buries the needle off the scale.. that was strange..but maybe not if it's dying..Anyway am I on right track thinking I need to replace this old alternator? It's likely not original to car but it is obviously an oldie..any comments appreciated.
thnx

That’s the wrong way to diagnose.

I will let the others tell you the correct way. Your car is not stock in the wiring and many variables to determine the issue. I wish you the Best to get it resolved.

thanks.

The output stud on the alternator should have the same voltage as the battery. If it is well over battery voltage and your battery is not charging I would suspect the wire from alt stud to battery is not connected.
Trace that wire out.

Iirc it goes through the firewall too ampmeter then back out to starter relay to battery.

The rest of the car has battery voltage?

eveything electrical is working. maybe amp meter finally fried as I have heard many do. I don't know. will try to trace the bat wire from alternator.