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What size opening from the firebox to smoke chamber is recommended (in square inches)...... I would say, you should weld the firebox to the smoke chamber.... Insert the FB about 8" into it, and have the top of the FB at the level you are planning to have the RF plate..... be sure the RF plate has at least 1.5 times the FB / Smoke Chamber opening under it for good air flow...
If you choose to have an external FB ducted to the smoke chamber, I don't know what to tell you...... A setup like that is more or less designed for a cold smoker because of the heat loss...... Dave
A member asked a question about my thinking and explanation above...... here is a drawing..... click on it to make it larger....
On the exhaust stack, I would have it below the top of the chamber. Either come out the side or extend it into the chamber from the top. My last build I had the exhaust on the sides and had no problem, my current build I extended the pipes staight down into the chamber 8" from the top/