Voltage Regulator and low output from Alternator

Did you ground the second field terminal?

Basically this is a simple circuit

The regulator MUST be grounded. MUST!!!

The regulator has two connections, IGN and FIELD

IGN goes to switched 12V from the ignition switch, and this is subject to "suspected" voltage drop

FIELD goes to either field terminal of the newer style alternator

The remaining field terminal must be grounded.