1967 100hp Johnson tach question

[Dadwith2boys]

I have a 67 100hp Johnson V4TL-13a. It is an electric start (not sure this motor is always electric start), but I would like to hook a tach.

I have looked at the http://www.tinytach.com/tinytach/gasoline.php but an confused how to wire from the engine (Spark plug wire) to the dashboard.

I have seen this installation guide: http://www.tinytach.com/tinytach/installation_gas.html but it is connected to the spark plug wire......

Thanks