Isn't there a fuel cut off solenoid on these trucks?
Failing that, fit a hidden switch in line with the starter solenoid.
Or fit some interlock wiring, eg, brake and clutch pedal have to be depressed, and err... windscreen washer has to be on, or what ever you like.
Or have an old fashioned dip switch on the floor... Keep it simple, you don't want to disable it every time you leave, far better to have a fixed starting procedure.:thumb2
Or consider a key code switch system.. eg enter a four digit code...
Rustic