It looks like a 48" bump out for the old fireplace. If so, that is a typical width. I'm a little surprised the chase was not insulated.
You might want to consider a BK Princess Ultra for that space. it will just fit in a 48" alcove.
If I can get the 52"-53" by removing the vertical double 2x6 and brick would the BK King work? If so its a done deal I'm getting the King. I haven't found anything on the side protectors/fan added clearance yet on the BK site. I really need as much heat as possible.
If so I can just put backer board and tile up for the alcove walls.
Say it will work!! haha