I'm not a gunsmith but I will respond with a
NO, DON'T DRILL THROUGH!!! The top strap is critical in strength and you don't want to compromise that!
Look into Jack Weigand (sp) mounts. I had one drilled onto my Ruger Redhawk for a Leupold/Gilmore Red Dot. It utilizes the rear sight screw hole and one other was drilled and tapped.
http://www.jackweigand.com/