Overcharging 73 Swinger

Yes- likely the alternator.
The green wire is the field's ground. The voltage regulator controls how much current is allowed through the rotor by that connection.
So the only explanation is that brush and terminal is shorting to ground.
It might be as simple as replacing the brush and insulator or the insulating washer.
I'm pretty sure I have a couple new ones. They look like this (might take a few minutes to load the pics)
Alternator repair, a little show and tell.