Unless your car had gauges, and not idiot lights then there is no tach pigtail to wire in. Cars had gauges or warning lights not both. If it had gauges then all of the gauges plug into the thick film (heavy duty printed circuit) behind the dash and a single connector wired them to the sending units.
That is why most hot rods you see have an aftermarket tach screwed to the steering column with two hose clamps. Few factory gauges where accurate enough or fast enough for racing.