I would use the Ford starter solenoid and set it up so that the wire running to the starter is only energized when the key is in the start position. Something else to think about is if you ever run it at a track then you will be required to have a remote kill switch that is accessible from outside the car. I think the small and light lithium car batteries are down to around $300 now though. Might want to consider going that route and build a smaller battery tray in the stock location.