doc, Is the auger pulley on the back of the blower housing inline with the small pulley on the engine? Is the auger pulley wobbly or bent? Check too that the spring for the brake assembly is still there and this underside "belt finger" is in place. Also it may be possible the rod, just below the belt finger I have pictured (where the blower housing hooks over) has cracked a weld during re-assembly? That could cause the blower to tractor mounting to wobble under load. Just throwing out ideas beyond it likely being a wrong belt.