JMR,
Yup, happened to me several times when I was riding that tank steel cruiser. My motor makes good power, I'd say much better than stock, I'm 220lbs, going up a hill, BAMM sam thing, broken mounts and studs etc... Good thing it pulled the spark plug wire off or it would have been much worse. The problem, at least for me was the extended front mount needed to mount the engine, the "U" shaped front mount was not on the frame, only the studs held the torsional load in the front. Switched bikes to one the motor fit well on both tubes, very nice. Less vibes, the bike is a suspension MTN bike, Thick wall aluminum, 200 miles on it and not a bolt loosened. Don't forget the blue loc-tight, anything loose will fall out. At least for me, those 2 6mm studs in front, even good aftermarket studs, broke. Just not strong enough. Hope that helps... you are not alone!
Mac