I would be very tempted to make U-shaped forms out of the 3/4-inch plywood that wrap around the concrete block and extend back on the walls about 1 foot. The pieces of plywood that cover the sides ...

Place a 16-inch-long drill bit into the drill and put the bit into the center of one of the hollow cores of the concrete block and drill down into the stone about 4 or 5 inches. If you are just ...