9 1/4" is the original disc size from '89 to '91(ish), when it was replaced with the 9 3/4". A lot of earlier cars were upgraded to 9 3/4" specs, but there are still some early cars lurking with smaller clutches. My black '89 still had a small clutch when it went out in 2002.
All SHOs after the switchover point will have the 9 3/4", unless someone got silly and transplanted a 9 1/4" clutch, pp and flywheel into a later car.
The flywheel is the same base piece for either clutch (same number cast into the wheel and everything). The only difference is that the three locating dowels, which are located about 1/4" further outboard for 9 3/4" duty. Switching from a 9 1/4" to a 9 3/4" (or the other way if you swing that way) requires changing your flywheel.
Not sure about the 9 13/16", though it's probably just a 9 3/4" that someone got too **** about measuring.