Older Fords use a separate external voltage regulator, a squarish metal box approximately 4x4x3 inches, mounted on a fender in the engine bay. I'm not familiar with Ford alternator wiring but your initial measurements seeing 12.6v on standby and 14v with engine idling suggests the alternator is outputting correctly despite the voltage gauge being incorrect. My guess is it's wiring between gauge and power source. A Ford service manual would help pinpoint wiring to check where the 8 volts is measured. In addition, the gauge ground may be an issue if power or ground connections are corroded.