How to install a new firem=place mantle to rock FP

I would like to replace the old mantle on my rock fireplace. The old mantle is a solid piece of pine attached with pins drilled into the rock.
I am building a set of matching bookcases to stand on either side of the rock fireplace and a new mantle out of one-bys and trim to span between the bookcases and over the fireplace. I need to know the best way to attach the mantle to the rock FP when I finish.
If you have done such before, please let me know how you did it.
Thanks, Richard Hollingsworth

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