Stuff like that is best tightened with a torque wrench or a small 3/8" drive ratchet....preventing massive torque.

Those motor mount bolts can be a real pain.

are you able to lift up the motor and remove the motor mount? If part of the stud is still sticking out of the arm you might be able to grab it with a vice grips and remove it.

Otherwise I'd go for removing the arm itself like has been suggested.

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