I remounted the alternator with a proper bolt & spacers and hooked up the voltage regulator. The voltage loss through the ground and the hot side was less than .04V for both conditions.

When driving around today I noticed the voltage bounces between 14V - 16V at idle whether the fan is on or off, and it's steady when the fan is on while driving. When driving with the fan off it was sometimes steady and sometimes bouncing.

My 6AL box is mounted on the firewall near the passenger's feet so I can barely hear it buzzing when sitting at idle. When the voltage surges, the buzzing surges with it, but the RPM stays steady at 850 (no vacuum leaks). Similarly when the fan is on at idle, it surges with the voltage.
I'll try a new regulator and see if anything changes.