Pipe from my fire box to Cook chamber, what should I use or what is safe to use? Heating and air duct? Black stove pipe? some other material?
Same question regarding the smoke stack?
In general, the thicker the better.
Steel or iron pipe is the way to go.
Stay away from heating and A/C ducting; it is too thin and you don't want anything that might be galvanized!
Black stove pipe is also too thin IMO.
The smoke stack can be thinner material such as black stove pipe or truck exhaust pipe.