I recently made some bookshelves over cabinets which flank the fireplace in our dining room; the job also included a new wider fireplace mantle which as was pointed out to me "was too narrow' to display anything. Guess where that comment came from?
the project was very successful in part because I built and assembled the units separately; there is a base unit which was leveled and anchored to the floor, attached to this is the 2 door cabinet console and above this is the bookshelf unit. the enlarged mantle board forms a bridge between the two bookshelf units and lies above the old mantle. I was able to match the molding detail of the new mantle board with the original so it is nicely integrated.