i'd install it just before starter, guess if we're talking slant six be easy to bypass, could hide a continuous relay on the main battery cable somewhere and have it control that.
Ah! That's a good idea. Then the starter can't be jumped at the relay. Slick.
And you could put the relays under the dash with two extra wires running out to the the relay. One to return from the relay and the other to connect to your starter solenoid with your kill switch relays between them. Should be fairly well hidden and clean looking.