You can also run a circuit breaker. They have some nice high-amperage models in the car audio world. That's the route I'm going with, because you can simply reset it or open it as a suplemental anti-theft type power cut off.

1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)