What's the cleanest way to wire a cut off switch

cut and dried way to go.

run the positive thru the switch and to the starter

from the starter run a 10ga wire to the starter relay post (this feeds alt power to the battery)

cut the VR feed wire in half (from un-resisted side of ballast resistor)

the bulkhead side will be routed to one of the small posts of the switch

from the switch run another wire and connect to the other half

what this does is:

cuts main battery voltage from the switch forward

and

cuts power to the VR using the switch which prevents run on.

for this to work you obviously need a 4 post switch (two large and small) but is the easiest way to go, no relays and BS to wire...