Well since everybody has a different meaning of stiff and its such a vague term lets ask these questions. Does the car shift fine? Does the clutch slip? If no, then I say its good. A bent fork would only really affect the pedal travel. If it was bent in, it would "slackin" up the clutch cable and not give enough tension to pull complete disengage the clutch. If it was bent back, it would probably lower your clutch pedal since its pulling on the cable. A stock clutch in a mustang really isnt that stiff. You'll have to elaborate on how stiff it is before we can tell you if something is really wrong. If its really bad, you could have a bad pressure plate but that would affect driveability I would presume.