The unfortunate truth is that anyone with a butter knife can steal your Z and it gets down to how badly you want to protect it. The Key systems in the 70's were really poor, and with 30+ years of wear a nail file will open the door of your car and start the ignition...So Z cars are easy targets.
My suggestion is a relay tied into your ingition switch that has a momentary button that must be pushed to start your car. This is achieved with a relay and a little intellegence and a hidden switch. it would require the would-be thief to have your key and know where this switch is mounted to defeat your simple alarm.
The disadvantage is that if they want to steal parts or are using a tow truck this will not help. Also, if you loan your car out often it could be a problem telling everyone how to start your car. Also, it is a known fact that most thefts are by someone you know, so if they know about your hidden switch it wont help either.
A good alarm is a good defense too, make it sound different than the thousand or so you hear on a daily basis as your ears will become sensitive to how your alarm sounds. In my personal opinion, the louder & more obnoxious the better.
Most 240z's are vulnerable around the battery, holes drilled in the bottom of the battery or the cover over the top gets pried up and the battery becomes useless to power your alarm. A battery backup siren is a good option for this potential threat with a standard alarm.
I hope this helps...