I used a similar setup, with 3 rotary switches with about 5 positions each. There are hundreds of possible combinations, like a combination lock. Just wire the switches in series, where all 3 have to be in the correct position for the ignition to work. The "Tach" terminal on a GM HEI can be grounded to disable the ignition, without doing any damage at all. That's how a tune-up tester shorts out individual cylinders while the engine is running during a power balance test. A simple toggle switch attached to ground and the tach terminal will work, but the 3-switch rotary setup in the power side is better. (or both!)
Jerry
