All good ideas, Eddie. However YOU don't know what is under the floor.
It appears that there is ledge close to the surface, but it never is level, so drilling and pinning is an option IF you saw-cut the center section where the new chimney would go.
The interior loop heating area lost is no big deal, but as I look at the structure (man, I'd love to have that one!), the gables are the only logical spots IF you would construct an outside chimney.
The lower side is the logical one, because you could have the chimney with 2 or 3 flues, look terrific, and have a flue on each floor.
Structurally, digging to ledge or digging to solid soil is possible, frost walls on a big reinforced steel footing is VERY cost effective, and building the chimney is straight forward.
I understand the desire to go internal here, but saw cutting, demo, excavation, re-pouring won't be cheap either. The 'outside solution' causes NO interruption in the plans for the inside either.
Personally I 'dislike' SS chimneys and at 45.00 a foot plus installation.......nuff said.
This in Maine; coastal Maine.....this is a permanent thing, AND masonry will allow for more than one flue.....at a small cost. I'll go snap a few pics of SS installs locally where they have 4500.00 in parts. OUCH!!!
A nice looking building deserves a nice looking chimney. Core a couple holes for the flues through the existing foundation, and clean-outs can be outside.
Masons are slow around here, keep you ear to the ground Tonto, something good is coming your way.
My thoughts on the floor touching the perimeter....and related heat loss.....well, lets just say you are heating with coal, and it won't make any difference.
The thermal mass you will have in this structure will just amaze you. If it were NG or Propane.....well, you are not that foolish.
I hear you are working on a couple hi=tech 'canoes' up there.....probably in a heated shop.....grrrrrrrrrrrr.
My other thought is that the chimney on the lower side would allow VERY easy access to coal storage, mechanical room, etc. and keep any associated mess off the living area.