If you're talking about the nut that holds the cylinder together, you better have a BIG cheater bar available. I spent an hour with my friend a couple of days ago trying to loosen that nut/fitting on his New Holland cylinder. He took a 36" pipe wrench plus heat, and could not budge it. Ending up making a tool from a 1" diameter pipe, plus the 10' cheater bar, and me on the torch heating the cylinder up. After multiple tries, it finally broke loose. (We literally boiled the hydraulic oil in that end of the cylinder)
Bottom line is do whatever you have to do to loosen it. It's tough to bend the shaft, and since you're replacing the seal kit anyway, don't worry about how much heat you put on it, even if you destroy an O ring or seal.