Will, if you have a pre-100 CAV you most likely have the Smith's electric oil pressure gauge.
The Smith gauges tend to last a lot longer than the sensors. So, I would guess it's your sensor which has gone bad. As you probably know, the sensor is located just forward of the oil filter on a SBF. Sensors vary, but typically they work by varying the resistance with oil pressure....with resistance (measured in ohms) in the range of 10 ohms to up to maybe 100 ohms. I don't know what sensor you have, but that's the typical range for many.
Personally, I would just install a new sensor and see if that fixes it. The sensors are usually inexpensive and easily changed. If that doesn't fix your strange gauge reading then I would think the next possibility is a grounding or short somewhere in the wiring between the sensor and gauge. The early CAVs had a few short cuts in the wiring....as I recall, the full voltage of the lights went through the switch rather than through relays, for example. This kind of thing can cause localized hot spots and melt downs....which can affect other circuits in the loom if wrapped together.
Hope this helps. It's not a surprise the fine fellows at the factory couldn't help - it's my understanding that almost all of the early cars were shipped without engines so the engine installer would be in charge of what sensor got utilized.