You can get the landtile Z with this code, and then move the "Caster" to the Z level of that X Y position.
LandTile landtile = Caster.Map.Tiles.GetLandTile(box.X, box.Y);
Caster.MoveToWorld(new Point3D(box.X, box.Y, landtile.Z), Caster.Map);
The box would be your Rect3d or whatever.