I have a diagram for a two relay kill switch but am not real sure on one of the connects.
[IMG][/IMG]http://www.synthetic-oilonline.com/d.cgi/5025806/home.html

The diagram is for a coil ign system however mine is HEI and I'm not sure if the positive connect would be the same as the connect for the tach. This setup will disconnect when the ign key is turned off and the button must be pressed while cranking to start.