This thing is a beast and it's gonna put out some good Q! Looks like it's made to be a carolina hog cooker. To use it as a true offset, the firebox is quite small. By my rough calcs 275 gallons = 37 cu-ft; you need a +12 cu-ft firebox or something like 30"x30"x30" to be safe. I'm pretty sure that is why there is a door to add coals for direct heat. I'm just guessing but it's pretty common to use this kind of cooker in conjunction with an external fire source where you can shovel in hot coals and spread them as kind of a low intensity direct heat.
Got a bunch of cooking toys and a custom metal fabrication shop where I spend my time building all sorts of smokers & outdoor cooking gear.