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You might think about installing a slip clutch on that mower shaft. I have a stump jumper and a slip clutch on my Woods M5 mower and have never had any problems. A slip clutch can be set to slip with any amount of resistance that you want and you never have to replace any shear bolts. It takes a heck of a whack on the mower to break a shear bolt.
Tractor Supply sells slip clutches.