Wow! Something is out of alignment for sure. I'd be checking for any broken motor mounts. Check for the proper 5/8'' primary clutch to secondary clutch offset, not to mention proper belt deflection also. There should be some side play on the jackshaft with the secondary clutch too. All this is figuring both clutches are in good condition. Another thing....once a belt blows, it can leave shreded material in the sheaves causing more trouble....not to mention a blown belt can also wreck the crank.