the 05+ GTs are hydraulic. i did a clutch install for Muscle Mustangs and Fast Fords back in September of last year. the 79-04 are all cable. i believe all of the mustangs before that are a mechanical linkage, except for the ones that people modified to a hydraulic setup.
btw most ford dealers, especially salespeople, have the same amount of automotive knowledge as you average teenager whose only automotive education is the fast and the furious.