Why? Is the clip going to take that extra height? Are you talking about a shim? You would be better off buying a non-conductive "cool-shim", I don't know where to get them.
As far as a solid copper block, the only reason to ever use another metal interface between your core and your hsf (waterblock) is if you are sandwiching a peltier inbetween. Otherwise you only end up with a second layer of thermal resistance.
Hope your talking about a shim....