[Yetiman]

Taking the discussion to the next level, how to keep the engine from running from the alternators power when disconnecting the switch...

If you put the disconnect on the battery's negative lead or on the positive lead, the engine will most likely continue to run if you turn the disconnect off while the engine is running. The alternator will continue to power the ignition untill you turn the ignition switch off.

I will also point out that SCCA and NHRA require the disconnect on the outside of the car to be connected to the positive side of the electrical system. Switching the negative won't pass tech.

There are a couple ways to wire your disconnect, and they depend on what you want to achieve.

On my own car the disconnect is on the positive lead from the battery. This allows me to kill power to the car quickly and easily while working on the car, and disconnecting the negative would work the same way and just as well. I am aware my car would continue to run if disconnected while running, but that is not a concern of mine.

If your number one concern is allow someone to shut off the vehicle from outside the car or to have A hidden security disconnect, simply wire it to the power source for your ignition. For this you can even use a low current (20 amp capacity) switch. You will still have 12 volts under the hood, but the engine will shut down and won't start again till you reset the disconnect.

If you want to have a foolproof way to kill power to the whole car beyond the switch And have the engine shut off when disconnecting you need a four pole disconnect (versions of which are readily available from most suppliers).
They have a pair of high current contacts for the battery wire (you could even wire this to the positive OR negative battery lead as you prefer) and a second set of low current contacts for the ignition system, disconnecting both circuits at the same time with one flip of the disconnect. This is the "right" way to do it.