I’ve rebuilt a number of cylinders of similar design on Case skid steers, no tricks to them at all. Never hard to get apart. Makes me think the threads on yours are rusted, although that seems unlikely as there is an O ring and backer ring between the threads and the flange of the head. Could be rust between the O ring and the flange edge.
Can’t remember now, those might be buttress threads. I would advise penetrant at the flange, and heat. Be careful with the heat. Also support the barrel right under the head when you try to crank on the head. I would suggest applying torque via the pin wrench, and at the same time hit the other accessible pin hole with an air hammer in the direction of rotation.