What the heck am I missing?[/b]
GCR 17.27: "The master switch shall be installed directly in either battery cable and shall cut all electrical circuits..." (my emphasis) I was describing above with this requirement as an assumption.

You don't have a choice: the battery must be completely isolated from the rest of electrical system. If you did it as you describe, then the remainder of the car's electrical system is still getting juice from the battery.

Remember: it's not JUST about getting the car to shut off; it's about getting the car to shut off AND have no charge to ANYTHING else in the car. Picture running fuel pumps while you're upside down, or battery cables chafing against smashed sheet metal with dripping fuel, or sparking instrument cluster wires 2 feet from your fasce, and so forth.

You will have to remove those two wires from the battery, attach them to a common block, run a cable from that block to the kill switch (or run the two wires on ring terminals to one side of the switch), then from there to the battery. Alternately, you can bisect the battery ground wire with the kill switch. THEN, you run either the ignition or alternator field circuit through the smaller terminals, and now you're golden.