Yes, you should be worried about your car being stolen! These cars have been hot theft items since the day they left the show room floor! It's any wonder why the dealerships didn't just keep them all under lock and key.
Fortunately I've never had one stolen, but I did have an 86 Cutlass Supreme back in '95 that was formerly my cousins...he said twice someone tried to steal it.
Hidden kill switches, Steadfast column guard are what you need. I recently did purchase a Steadfast....have yet to install it (thinking of power coating it) but it pretty much doesn't easy come back off once installed.
Kirbans sells a "mystery" theft device for G bodies....for like $25.
Keep in mind nothing stops a tow truck short of you capping the driver thief dead between the eyes.