Trunk mounted battery

Actually you don't HAVE to change anything................

But If you want to utilize the 4 terminal switch, it would / could save one large gauge wire "to the rear."

Move the alternator output to the start relay battery stud. This elimates that large gauge wire.

Run two no 14 wires from front to rear to the small switch terminals.

Use those to break the control line (blue) to the CD relay. You now would not need a CD solenoid, you could use a smaller Bosch style relay

Use the relay contacts to break either the alternator field or the ignition system.

If you have an MSD ignition, you can also eliminate the CD solenoid / Bosch relay.

You could use the small disconnect terminals to break the small red control line to the MSD box

Any of this will cause the engine to stop.