What kind of transmission do you have? Do you know the diameter of the clutch disk or flywheel?
Luk's online catalog lists one for a '69 Bronco with a 200 but it says 10-5/16" disk. Sounds too big. As far as I know there were only 2 sizes of clutches available for the 200. 8-1/2" up to 1966 or so, and 9" thereafter.
If you have a 3.03, a T-10, or a Toploader transmission, and a 9" flywheel (I would bet that's the case), then you should be able to ask for a clutch set for a '68 Mustang with a 200.