You could put an immobilizer in the starter circuit or maybe the fuel pump wiring, this will stop 99% of thieves, my favorite antitheft tech is whenever I have to leave my car in an exposed area is to whip the rotor out and put the cap back on, small and easy to carry...
When i think of who would target these cars i think of young people joy riding, if it cant start then its not going anywhere, i really cant see a reason to steal these cars unless its at the extreme end of performance mods... although as they become more collectable this may increase.. some cheeky crackhead stole my external visor off my car when it was sitting in my driveway, didn't bother trying to start it or go thru the car itself, change in ashtray still there and stereo... Hmmm