Looking at your dedicated woodstove space, I would say it is totally doable to replace it with an
RMH. The one possible issue would be weight - a viable RMH would have to weigh around a ton at minimum, to hold
enough heat to coast overnight. What is the foundation like for your house? How practical would it be to
beef up the footings or add new in that corner?
To start the layout discussion, I would suggest putting the RMH
feed and barrel about where the woodstove is now, with a bench extending to the wall behind it and wrapping around to the 5' side. This would actually give you a nice seat to relax and keep warm on, where there is little usability of that corner now.
I would think a 6" system would be the right size. This can use a 35 gallon barrel instead of 55 gallon, saving space. It would have a smaller cooktop, 15" diameter vs. 22" for the 55 gallon. You could put in a back shelf for warming/simmering if you wanted.