Hi Louis, Firstly, anything is possible! Now, whether its practical/ legal/ efficient etc. is a whole other story. There's quite a bit of info that we're going to need to be able to help you out better....
- Without any info on what kind of floor you have, how its supported and how much mass your thinking of, its hard to start with any recommendations.
- I'm assuming the old fireplace has a proper chimney that your going to utilize for your exhaust.
- In order to get a window to see the fire, a batch box
RMH is the most common way to go. Depending upon your skills, it is harder to build than a J tube but if your confident then it can be done.
- Do you have a copy of the
RMH builders guide? A great book that covers a lot of info all in one spot.