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.
Weber 26.75 OTG w/CI grates & SJ/WSM Mini
Home built 24"x72" reverse flow stick burner trailer with 18"x44" grill w/CI grates and upright SS gas oven/smoker.
Home built 48" fire pit with a 30"x30" Santa Maria style ranch grill.
Home built lump charcoal retort, iGrill & a bunch of other cooking toys.