Nice looking car. I don't think you are being paranoid. I don't drive my car to places I have to leave it for long periods of time. I park it where I can see it from a restaurant, etc. There always seem to be enough people looking at it and I think it would be hard to steal when I am out and about. The more probable scenario is having it stolen from my garage. I usually disconnect the battery cable when it is parked in my garage.
There are alarm systems available - check the Year One or Classic Industries catalogs.
The easiest is a hidden switch or one of the clamping devices for the brake and/or clutch pedal.
