Engine Adjustment

Is there any way of moving the engine forward on its mounts? I am trying to get a little extra bite to the deck belt. I am having belt slip, and yes, I have the OEM spec belt on it (just changed it). I was considering unbolting the engine and slotting the mounting holes, or shimming the front mounts. Would this work? Any other remedies?

Thanks.

edit: I was also thinking about a shorter belt. One that was 1/2 to 1" shorter.