I want to mount an aluminum sculpture to the wall over a fireplace. The wall is covered with stone facing and mortar, over plywood I believe. The sculpture is about 40lbs, and provides two "keyhole" tabs to mount it. I've located the sculpture so that the tabs line up with a mortar joint, drilled two holes, and ratcheted in two Tapcon 3/16 x 2 & 3/4 masonry screws. However, I have two problems: first, because the stone face has varying depth, one of the 2 & 3/4 screws isn't quite long enough. Second, and more importantly, I'm not sure the mortar is strong enough to be the only thing holding the screws - and maybe I should be using some anchor.

Any ideas? I feel the drill go all the way through the mortar, so I know there's hollow space behind. I could put in a drywall type "wing anchor" which would spread out, but that would mean making a hole larger than the bolt. I know there are sleeve anchors that spread out, but would these work in the mortar, and would I need to know the depth of the material? And, to add more thought to the problem, should I even be bothering to line up bolts, or should I be fastening the bolts snug and using piano wire with hooks, which provide much more play.

Thanks