Still shouldn't broke..the studs if installed correctly would strip/pull out first before the mounts broke. The reason they broke was as I stated they were not installed correctly. When mounting a engine, the rear mount has to be mounted first, this mount has to be flush
and tight enough (60 to 70 INCH POUNDS) to the tube, so the engine won't move. After that the front mount needs to be installed. The front mount needs to be flush with the tube and if not a fabricated front mount made or shims used. Installed this way it's impossible for the engine to drop. I don't care what your credentials are. So I say again, the engine wasn't installed correctly... Take it for what's it worth.
Illustrations of mounting...
Engine Mounting pictures by Ron-Becker - Photobucket