If there is room to do so, try putting a stud, not a bolt into the output shaft, and secure it with a thread lock liquid like Loctite. Then use a nut to secure the universal joint. That way there won't be as much stress on the thread in the shaft as it will be a "one time" installation and the nut will be the part taking the tightening stress,
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!