Looks as if your lid/door for the Cook Chamber does not seal well.... That is a MAJOR problem trying to control the temperature...
Is there a damper between the FB and CC ???
If the smoker is sealed air tight, meaning no air leaks, you do not need any adjusters other than the 2 air inlets on the FB....
Generally, run the upper air inlet open enough so the FB does not get HOT, which means the heat from the wood is heating up the CC and not the FB...
Adjust the lower air inlet to adjust the heat....
I really don't think 1/8" steel for a 300 gallon cooker, is doing you any favors.... Add welding blankets to the CC to help insulate it... It might take 2 blankets thick... If you want, you can "skin over" the CC and blankets with 18 gauge steel sheet... To protect them, hold them in place etc... Secure with self drilling screws to the CC.... and the door... etc.... until it's totally covered...
I agree with Dave..... Also Like I said earlier try putting a plate over the stack and regulate it down. The hot air might be directed right up to the top of the cc and out the pipe.
Boykjo