Check the Ign. terminal on the regulator. It "senses" the available voltage and energizes the field windings in the alternator to bring the voltage up. A low reading at the terminal will cause an overcharge condition.
Next I would check the reading across the battery. it "should " be very close to the ign terminal reading.
if not try removing and reinstalling the bulkehead connector at the firewall and repeat the above tests
Report back and we'll got from there