Congratulations on the car! Your first Vette is an awesome one!
Jumpy speedo is usually an old, kinked, and/or dry cable. I believe the '69 cable is two pieces. You may want to disconnect it at the transmission, hook up a cordless drill to it, and spin it. Do it again at the halfway point. It may tell you that the cable needs replacing; you may even be able to hear it making noise as you spin it (not good). You need to spin it backwards for it to register. Depending on the drill, you'll only get about 20 mph.
The speedo being off could be a number of things. Someone could have changed the rear gearing or swapped transmissions. Or it could be just a fubar gauge.
Odometer: no clue. If the cable isn't your problem, you may have a bad speedo.
As far as the voltmeter goes, if it's a regular voltmeter it will have nothing to do with your voltage regulator. It'll show whatever buss voltage is wherever it's hooked up. However, with the car off, it should read lower than with the car running to show that the alternator is doing it's thing.
Removing the speedo is not a trivial exercise. You'll have to drop the steering column, remove/loosen the center stack, lift the dash pad, then pull the gauge cluster. Then there are the 15 or so light bulbs plugged into the gauges, as well as the vacuum lines for your headlights and wiper door. Which is a long way of saying that if it was my car and I could fix the jumpy speedo with a new cable, I'd live with the broken odometer.
Good luck!
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"Let them that don't want none have memories of not gettin' any."
- Brother Dave Gardner
