Check out this Pelican tech article for what might be a simpler solution.
http://www.pelicanparts.com/techarticles/Mult_ignition_cutoff/Mult_ignition_cutoff.htm
I don't know if the tach trick would work with your model (although it seems simple enough to check), but the fuel pump kill switch idea might be a good route.
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Jack Olsen
1973 911 T (3.6) sunroof coupe
jackolsen@mediaone.net
[This message has been edited by JackOlsen (edited 02-11-2001).]