Is there gap in between the mount and the tang on the engine. Can the engine wiggle back and forth or is it tight fit. I would use washers to tighten the space between the mount and the tube on the engine side of the mount. This should allow you to tighten the bolts down so it doesnt move.
If your looking for smooth ride get stock mounts. Other wise tighten it down so its locked in place. It should lift a fender when it revs not just wobble around inside the engine compartment like a stock engine.