Why use an old fashioned masonary chimney at all? I would use a standard triple wall stainless steel chimney. These only need 1" of clearance to combustibles and they snap together in minutes. Also, if you use a manufactured fireplace then you don't need all the heavy weight of a fully brick structured fireplace. This is an important consideration, especially on a second floor without having to beef up all your floor framing structure. The above could be done with a wood burning unit or gas. If you use gas you could use direct vent and just go straight out the side of the house with no vertical chimney. I achieved a beautiful, old traditional look using a manufactured wood burning fireplace. I bought a decent heatilator unit (heatilator.com) - not the best but not the worst brand - and I got one with no fan or vents on the face so I could bring my old fashioned tile or brick facing right up to the edge of the firebox. No metal shows and it looks 200 years old. I had a nice mantel made and done! I wouldn't even consider a masonary unit and chimney in your instance.