Ok, here's what i've got, a 250 gallon propane tank that someone already cut into. I'm needing to figure my firebox size but am having trouble with the calculators on here. My smoker main body is 30in diameter x 66in. long. I was planning on building my firebox 30w x 24h x 24 long. Also how big does my box to smoker opening need to be and how long should my exhaust be? Thanks!