Voltage Regulator?

The other issue which I forgot to mention is the ground side. Just like the "hot" side, the ground side of the VR MUST be right at 'same as' battery NEG potential (voltage) You can add grounding wiring between the VR and battery, what I like to do is add a no4 starter cable (eyelet to eyelet) from the engine block to a good firewall ground, such as the master cylinder mounting stud

On the V8s there are unused holes at the rear of the driver side head

This really sums it all up. For the VR to properly regulate at the correct voltage, both the ground, and the power connection MUST be at "same as battery" voltage. Current through a circuit, and too small wire, bad connections, or switches with deteriorated contacts all add up to drop voltage when current flows