This overcharging (over voltage) IS ALMOST ALWAYS due to either voltage drop in the harness or ground path voltage drop between regulator and battery. I've posted on this hundreds of times
In rare cases this is caused by the regulator, and also rare is a battery going bad
To test for harness/ ground voltage drop:
1....Turn the key to "run" with engine stopped. Hook your voltmeter as close as you can get to the bulkhead feedthrough the "ignition run." This connect to, amoung other things, the VR blue, the alternator blue, and the key side of the ballast resistor
hook your other probe to the POS battery post. You are hoping for a very low voltage, the lower the better. More than about .3V (3/10 of one volt) means you need to start looking
The functional path for the VR blue wire voltage is Battery........starter relay stud.........fuse link.......bulkhead connector.......ammeter (RED)........through the ammeter........out on BLACK........to the welded splice........branch off to IGNITION SWITCH connector.........through the switch.........back out the switch connector on blue "ignition run"......out the bulkhead through the firewall...........and to the under hood ignition loads.
ANY OF THOSE POINTS can and will cause drop
2...GROUND...........run this test with engine running at "low cruise" first with all loads off and again with lights, heater, etc running
Stab one probe directly into the mounting flange of the voltage regulator..........Stab the other into the battery NEG. Again you are looking for a very low voltage, zero is perfect, the lower the better.
