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.
As bdcardinal says, all the Fox chassis clutches through '04 are cable operated. And it's a crappy setup. The best thing that can be done is to throw away the cheap plastic self adjusting quadrant and install a solid one along with a firewall adjuster.
I had to rebuild the hydraulic clutch once on my old Triumph Spitfire. Not sure what this adds to the discussion, but oh well. Oh! This was 20 years ago in Logan, Utah. I went to a local "no-name" parts store to see what my options were. To my shock, they had a rebuild kit right there in stock! I thought I had the only Triumph in three states! I guess they needed frequent rebuilds.....