As you may or may not be aware, to kill the MSD unit (I have a 6A) all you have to do is ground out the white wire. For my application, the white wire was unused and can be easily extracted from the MSD wiring bundle.) I just installed a simple heavy-duty toggle switch (radio shack) between the white wire and a good ground. (This will allow the engine to turn over but never start - a big PITA if you forget that you have the switch on!) The white wire itself is only about 18-20 guage standard automotive wire. I used a slighty thicker gauge to connect to ground. Hope this helps.
Another approach is to break the 12 volt wire that energizes the MSD box. That wire is a 12 volt switched wire and w/o juice there the engine cannot start. I know this works cause I disconnected the one on my Vette when fooling with the distributor cover. Lots of cranking but no fire in the hole.
This is a very low voltage wire and it only signals the box that the ignition is on. I am NOT talking about the 12 volt wire that goes from the battery positive to the box. Leave that alone. That wire actually operates the electronics inside the MSD box.
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