It's strange how trucks vary so much, Usually title goes with the frame because it's the most significant part and the cab gets treated as a replaceable part. But there's also the argument that you can swap a damaged frame and keep the VIN and title for the rest of the truck.
If I was that guy with the cab I'd want to keep the title too but wouldn't necessrily use it. At least if the problem comes up why a warlock has a chassis that's 40 years newer there's paper work to go with it and not that someone threw their cab on a stolen new truck.