Our mantle is a reclaimed barn floor joist. It weighs aprx 100-150lbs, measures 8" tall, 15' deep and 5' wide. Our builder tried to route out the back to insert wood blocks to secure to wall, but the beam is so old, it started falling apart. In a bid to salvage the piece I spent so much time, energy and money on.. this is what we've come up with to secure it to the wall.

Unfortunately, even with these 3 brackets, it's not secure enough. I contacted a custom metal fabricator with the idea of doing iron corbel/brackets that wrap all the way around the beam, attaching to studs on the top and bottom. Only problem with this is it would have to be built to accommodate the rock around the bracket and I'm not sure how they'll do that. Any other ideas to make sure this thing doesn't come down?