I don't think a switch on the ignition wire is any good. Who is going to mess with the real ignition switch to start off? there is no steering wheel lock so why bother?

A theif simply runs a hot wire from the terminal on the starter to the coil, pushes the foot starter and he is off.

If you are to foil that plan the hood must be locked, then a hidden ignition switch could be usefull.

Or a hidden battery disconect switch that is hard to bypass and needs a key to operate.
To get around that would take a set of jumper cables to steal. Or a battery pack with the jumper cable ends to run the truck on long enough to steal it.

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-