I chambered my 275.Hi,
Just finished the fabrication on my 275 gallon oil tank smoker. I divided the tank into two chambers with some heavy gauge sheet metal between, and a series of holes drilled in the lower half of the divider. The top of the tank is also fabricated so all smoke has to go from the firebox side to the smoker chamber and is diffused back across the top to a 4" heavy wall steel stack with a damper on the top of it. This allows a firebox that is roughly a hundred gallons in size, and a smoker chamber of 135 gallons. I have two racks in the smoker box, they are 28 inches wide by 25 deep so it allows for a great deal of cooking area. The upper rack removes to stand turkeys and chickens and smoke them.
I just did some test runs today and can regulate the temperature anywhere between 165 and 275 degrees.
My smoker chamber has a sliding door on it that I fabricated by cutting the tank then using some angle iron and flat bar. The firebox is a hinged door, with an air damper below it.
I hope this helps.