Glenn Herbert

From the look of your plan, with the chimney rising straight through the central peak of the roof, I would expect a 6" chimney to work. This assumes that you don't have weird atmospheric or environmental conditions.

An important thing to be aware of is that a J-tube system has much less power for the size than a batch box. It has been found that an 8" J-tube is roughly equivalent to a 6" batch box in rate of heat delivered, so using the bell size for a 6" batch box needs an 8" J-tube to heat it adequately.

My experience is that in a space/chimney with good natural draft, an 8" J-tube is well served with a 6" final chimney leaving the bell.

Benjamin - I would make it a priority (if you don't already know) to get the current chimney draft situation clarified, as that will affect how functional the different options are.

That setup is known as a "plunger tube" and used where it is not convenient to exit the side of the bell. It has an advantage that the height of the tube bottom can be raised or lowered to fine-tune the amount of heat extraction and the temperature of the chimney exhaust. The upper part of the tube needs to be insulated to reduce heat leakage from the top of the bell.

I used to do that when I lived in a little travel trailer for 'mumble' years while building my house... the insulation behind the aluminum skin was so thin that a cave would melt out around the skin within a couple days.

My father used to collect leaf bags put out by people and pile them around the walls of his living space. Great insulation, and after a few years, what was in the bags had turned to rich garden soil.

Something that can affect practical core location is how good the draft in the chimney is. If it draws by itself in weather where you might want a fire, you don't need to worry much about a bypass. If the draft usually needs help to start, you want a bypass.

Yes, a stratification chamber or "bell" is just a hollow brick box with the combustion core feeding into it, and the exit to the chimney leaving from near the bottom of the cavity. The new hot air rises, gives its heat to the brick box, and falls as it cools. The coolest air ends up at the bottom where it leaves for the chimney. The internal surface area of the box needs to be sized to extract the right amount of heat for best operation, per instructions at batchrocket.eu.

I love the Japanese pullsaws, which can take just a paper-thin kerf out of the wood. As long as they have good guide slots at either end of the stroke, you can get a good board fairly quickly and then plane it to a perfect finish.