As long as it bolts together and works, no problem. I've actually used Ford pistons in a Chevy engine by honing out the Ford piston to use a Chevy wrist pin. (I did that for compression-height concerns with a stroker crankshaft). The parts don't know where they're going! I do find it unusual that a Ford-marked clutch disc would have the same input shaft spline as a Chevy, unless maybe it's intended for one of the B-W 5 speed overdrive boxes that the Mustang runs.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!