Put these on the fenders. They are nearly 100% effective.
You can jump one wire from the battery to the coil and jump the starter relay to start it on most of these cars, so ignition cutouts are pretty much pointless.
Fuel cutout are a little better, but don't stop towing.
Locked brakes on all four corners is better but not really feasable to implement. (blead down from premature rubber parts failure would not work out well)
Or you could go buy a 12volt security PIN pad and have it control the master disconnect solenoid in the trunk mounted battery box. No 8digit code you get nada.
Andrew
You REALLY should take a look at their coverage map, which even in the heavily populated Portland/ Ugene area, has a LOT of "blanks"
http://www.zoombak.com/coverage/
Neat idea. But it really isn't all that much better than a hidden kill switch. If you find it, you can still bypass it. And if they never start the car (and the professionals aren't starting them most of the time), then it isn't an issue.
Same thing with the steering wheel. Yeah, you need it to drive away. But if you're not driving it, it's not an issue.
A cheapo hidden micro switch will defeat the guys that need to start the car. A back up fuel kill will defeat the smarter guys that still need it to run. And if they don't need to start the car, you probably won't stop them. Think repo-men and tow truck drivers. They don't need the keys, they don't need it to run, and in a lot of cases, they don't even need it to roll.
An indentation of where my .45 sits on the dash and a couple of empty shell casting on the dash doesn't hurt/works fine for me so far.
GPS, kill switch, good insurance.
No matter what you do if someone wants your car bad enough they will get it.
GPS and a kill switch will help, but stated value classic car insurance is dirt cheap. Just make sure you have enough coverage to go buy the next one and sleep well at night.
With one exception, you want agreed value not stated, huge difference.