Cubs were made to be worked on with a few simple hand tools. Never seen one where you needed to modify a wrench to loosen the fan pulley. It's been a while since I have loosened one but 11/16 is stuck in my mind. What ever size it actually is, remember, DO NOT turn the nut on the rear end of the shaft. There are flats on the shaft and you put an open end wrench on them and turn the shaft. I would put a box end wrench on the nut to hold it but again DO NOT turn that nut. Oh, and one more thing DO NOT turn the nut on the back of the shaft.
Yes, 13/16" and I ground one down to fit.
I agree with Scrivet...DO NOT turn that nut.
I agree Don't Turn the nut. I tried!