To stop thieves I use a power cutt off switch. Second, a gasoline cut off switch.If you are using an electric gasoline pump, a separate switch to give / cut off power to the pump is the way to go. If using a mechanical pump, a motorcycle gas tank valve does it. With both devices, the thieves will never start the car ( no power) and if going through the power block ( which I doubt ) they will run out of gas within a mile.
Add to that an alarm , and if you leave yourcar in a driveway ... anchor it. Put a metal ear in the concrete of the floor/ driveway. Thick chain and a hefty lock from floor ear to the rear axle will keep it there.Short chain, so there is no area for busting the chain.
This is too much, the thieves will never have the time nor piece of mind to go over all those obstacles.
If you want to add a conventional alarm on top of it, fine.

O forgot to add, with todays technology you can add a miniature camera , hidden in a corner of the car.The camera is fed from the car battery, with ultra minimum power usage. I have seen this on TV, in reality programs, this cameras can be checked via computer.....I will find out about it. That would be cool, to tape those imbeciles while they take off......if they ever after all the security devices... but still they deserve hell for entering the car and violating it's sanctity.