Low charge to no charge issue pulling hair out
That's between 10 and 20 amps, my 67 will go there for a few minutes after a hard start. But it will settle down once the battery is happy.
My battery is 7 years old, garaged in northern Colorado, 1000 to 2000 miles per year.
I do think you have an issue either with the wires and connectors between the alternator post and or the battery.