Thanks for the advice, the wiring diagram was helpful, although it says to connect the wire to the positive terminal on the coil, but it only worked on the negative terminal.
Now I have another problem, when the tacho is plugged into the coil, there is not enough current to make the engine fire. Is there a way to get around this?
In response to having a round tach in a square hole, I am planning to remove the clock and mount it behind the hole, and only a small area will be obscured.

Tyrie