Unlike fireplaces built today, which are either prefab or which are built according to known, predictable patterns, old ones were custom built. I think, therefore, the feasability of the kinds of alteration you are thinking about would best be judged by a trained eye. On the other hand, if your's is part of a subdivision of similar homes, ask neighbors about their fireplaces.

If you haven't had a chimney sweep clean your chimney yet, which you should do, find one who is technically knowledgeable and, while he or she is removing the sot from your chimney ask him a few questions. If you've cleaned your chimney already, ask a local fireplace dealer for a visit and a free estimate.

Thanks

You COULD apply the wood right over the current mantle, using the correct construction adhesive. Power Grab might be a good one as it is OK to use on masonry.