At the open end of the 3" crank the keyway is .2360" when measured at the bottom of the keyway.
The PTO clutch keyway is .3765 measured the same way.
Clutch bore and bracket depth is 2.5" but the adapters I have found all go for 3" due to aligning the clutch on the crank and keeping it lined up with the mule drive.
Odd measurement on the crank, it's actually equal to 6mm. No problem, I'll just make a key that fits into the clutch, and is tall enough to fit through the shaft adapter, and cut the other side to 6mm wide. I might be able to get this made today!