As most have said ^^^ above, you can only slow down the opportunist or ` Joy Rider` types with a pedal lock or an ignition immobilisor or even a fuel shut off so the bus runs out of petrol, locking them in gear with a locking shifter or other means, chaining up the steering wheel or a steering wheel lock. Electronic solenoid type door locks are a good one as well as they aren`t expected on one of these.
Where it goes Pete Tong, is when they hook it up on a spectacle or drag it up on to a low loader, with a decent gearing on a winch it don`t matter if it`s in gear or if the steering is on full lock because they aren`t interested in looking after your pride and joy, they just want to drag it away for scrapping or similar .
Various types of trackers are available so you can locate it once it`s been nicked and find it again theoretically ,,, BUT !!!! Any thief that `specialises ` in older vehicles or any car thief worth his salt now days, will have a blocker for these trackers so you can`t use them. It does sound a bit hopeless but all the above are worth doing, and if you can put enough obstacles in the way it may make them look elsewhere. The only 99% guarantee is to make the vehicle untouchable by blocking it in with vehicles etc but remember if you moved them, then so can someone else, and keeping it out of sight in a lock up is possibly the best solution although if you locked it up, then someone else can unlock it.
Ozziedog,,,,,,,,,,,,,,,,The harder you make it ,,,,,,,,, the better.