Not sure how you guys can jump to that conclusion.
One likely common regardless of the cause is that the battery is low. Then when the alternator output increases with rpm, the battery sucks all the juice it can get. That's why you see the ammeter indicating high charge.
First - Slowly charge the battery on a charger to reduce the chance of overloading the charge circuits wires and connectors.
Possibilty 1. Battery is not getting charged at low rpm because the output is weak. This could be one bad diode and thats why with increasing rpm the alternator output increases.
Possibility 2. Battery not getting charged at low rpm because the alternator's field is not getting enough power at low rpm.
A loose or oxidized connection in the field circuit is a possibility too. This includes the internal ground that completes the field circuit.
Test if providing field power causes the alternator to produce power at warmed up idle or a little above. That is jump a wire from regulator in to regulator out and bring the rpm to 700 or so.
The original regulator is points style. Easy to open up and see if the points are sticking, dirty or burned. For the first two, clean up with a piece of paper or fine abrasive like crocus cloth.
Shopmanual will have all the details of adjusting.
Chrysler Tech series may be useful as well.
http://www.imperialclub.org/Repair/Lit/Master/index.htmLook at #169 for example to see the voltage reg
