Battery cut off switch
I don't believe the diagram is convoluted, nor are there a "bunch of relays."
TWO wires front to rear, and ONE relay/ solenoid up front.
You can use the relay up front to break the ignition path to the "what was" "dark blue" lead, that is, break the "ignition run" use the wire from the ign switch to power the relay, run the other half that fed the igntion, regulator, and alternator field to the relay contacts. This way, EITHER the ignition switch or the battery kill switch will kill the ignition. If you later go to a "one wire" alternator, you will be all set up.
Here's the deal: If you run a no8 cable from the battery to the alternator, YOU DO NOT HAVE a disconnect. What you DO have----is a large hot wire from the battery to the front of the car, hooked to the alternator!!! Some of the other diagrams floating around also show a second or even third solenoid.
If you leave the no8 run up front "hot" I don't believe it will meet the letter of the NHRA rules