Bottom of the heap for Shelbys, just a J code 302 with an aluminum intake and Holley (at best-iron intakes and Autolites also used) It should have an extra Shelby VIN plate riveted on top of the driver’s fender apron. Some very early cars have a dual serial number system that was soon abandoned. It will have the full Ford VIN plus 5 extra sequential digits. You can also run the Ford VIN that’s on the windshield base and door tag and run a Marti report if that Shelby tag is gone (though you likely won’t want the car if it’s missing). Coralsnake.com is a great 68 Shelby resource.
http://www.thecoralsnake.com/Productionchanges