SRS, we also re-purposed an unused fireplace. In our case (it was much wider than yours), we set two bookshelves on the hearth, and put air-blocking material between the bookcases and the opening. The rest of the hearth not covered by these holds additional books-- binders from our Sat. nite music program.

I think we lived here about 12 years before we went ahead and did it, and that whole time I think we used the fireplace once. Fires are nice, but only new one fireplaces with today's building codes create more heat than they lose. So our recreational fires are now fall and summer outdoors campfires.

~Susan