If there is a gap between the old mercury filling and the tooth, I would remove the whole filling while paying particular attention to the pulp of the tooth. I would consider placing a Fluoride releasing liner over the pulp of the tooth if there is recurrent decay. That adds protection to the nerve of the tooth. Then do the bonded new build up on top of it. That is what i would do before the final crown preparation.

Ammar Mekari