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I can bypass the startup wire interrupt with a screwdriver. So can a thief.The best wire to interrupt would be the power off the junction block on the firewall or the main 12ga circuit 2 power wire at the starter BAT terminal. Even if they can start the engine with a wire to the HEI and a screwdriver in the starter terminals the cops will notice no taillights.
Should be the 12ga purple wire. If you look at the wiring diagram for 1977 Chevy in Tue tech pages,http://forum.73-87chevytrucks.com/smforum/index.php?topic=30115.0 , you will see that the solenoid power to the starter is just that. Trace the line in the large diagram at the top of the PDF to make sure but that should be it.