1972 Plymouth scamp charging and grounding issue

Mmmmm, OP , seriously sounds like your new alternator and/or voltage regulator are not working and the alternator is not putting out any charge. The left-of-center readings on the ammeter all point to the alternator not putting out any charge.

Does the ammeter deflect to the right of center at all if you increase the engine speed from idle to fast idle with nothing turned on? It should show some right-of center- deflection; the more discharged is the battery, the more there will be right-of-center deflection.

Get access to or buy a voltmeter. It will be your friend on this car! Check the voltage across the battery at fast idle and warmed up, it should measure in the range of 13.7 to 14.3 volts. If it is much lower, then the alteranator is not charging. If you do this, please report back with the voltage that you do measure across the battery at fast idle.

BTW, a bad ground of the voltage regulator case to the chassis will cause the alternator to not charge so check that while you check all else in the grounding of the system.