I'm sorry I left that detail out also. I was going to use the pipe for the intake from the firebox as well. My fire box is about 18" by 18" box. Do you think it would matte a whole lot between 6" or 8" pipe?
gbpb, IMHO you could use either as long as you have enough stack opening to allow the airflow to flow freely. Figure how many square inches of opening you will be allowing to come into your firebox through your vent and use a stack which is equal to or larger in area for a stack. Hope I worded this so it is understandable. Just trying to help. I have a damper in my 6" stack on my woodburner but only use it sparingly to help with heat in the smoker. Use vent on firebox to control heat. My smoker has an internal firebox and is real easy to get hot. Good luck with your build.