Do you already have all of the 6" stovepipe for the mass run on hand? If not (and even if you do), I would advise making a bell instead. For much of the bell you may be able to use local rock mortared together. What is the character of your local stone? Flat, jagged, rounded? Unless it tends to all be smooth and round, you can build a secure masonry box with only mortar to import. Given the relatively constant heating load, a thick walled bell would be appropriate and keep the heat output fairly constant. You would need some firebrick for the top third or so of the bell surrounding the heat riser, as that sees extreme thermal stresses. If you use the traditional barrel around the riser, you can get away with just the core being firebrick or other specialty thermal performance material. What I did (for about 800 sq. ft. in upstate NY) was a brick and cob bell 6' high, with a (not yet built) 6' bench bell extending from the side. The bell walls are about 9" thick, and when heated up, keep warm for up to 24 hours.
For a 6" J-tube, I think you would want a bell with interior surface area (not counting floor) of around 30-40 square feet. A 6" batch box has been calibrated to want 57 square feet ISA, and J-tubes put out heat slower for the same size system so need smaller ISA to leave enough residual heat for chimney draft.
You could make a masonry bell 16" square x 4' high inside, for about 23 sq. ft. ISA, and a bench with interior 2' x 5' x say 6" high above the exhaust opening, another 17 sf ISA, totaling 40 sf. I would want Peter van den Berg to weigh in on this estimate. I think the constant cold climate would aid draft enough to warrant the large end of the range for ISA.