I'd get the o2 sensor(s) installed soon. You're probably running very rich without them. As for the tach, did you try the general tune-up stuff? Plugs, Dist. Cap, Rotor, etc.?

If the car seems to run ok, it might just be a wire/ground issue with the tachometer. I'm not sure how to check it with a volt meter. Mabye somebody else could suggest that?