[LH Lead-Foot]

When saying battery gauge are you talking about the "Amp" meter? I would check to "F" terminal wire to the alternator after switching to a 12SI (With Fan) to make sure it has resistance using a test light. It should be dimmer than normal. If dis-connected, you can measure with a volt/ohm meter.
A 460 ohm resistor is installed between the ignition switch "ON" terminal to protect the voltage regulator, this protect the voltage regulator from current, while proving voltage to turn on with three wire alternators. Once started, the diode trio provides field voltage to keep the alternator turned on. If a diode trio problem occurs, one of three diodes will reduce the output voltage. This is why the dash light comes on dim and a little brighter when revved up.
The stator is a "Y" connect type and the field terminal is dead when not turning. When the engine is running, voltage is produced and matches system voltage. A this point, you have B+ on both sides of the idiot light, with no potential for current to flow, the bulb goes out.
The common practice in the 70's was an idiot light for charging using a 194 peanut bulb. Both the bulb and resistor acts as a current limiting device to the "field" terminal for turn-on signal. Typically used in parallel to the bulb & resistor, as this practice allows the alternator to stay on, if the bulb burns out. Also, since the "F" terminal is dead when off, this was typically used to control a choke relay coil. With voltage, engine running, the relay is energized and supplies the electric choke with voltage to heat up, usually from a fuse on the ignition switch on side.