Your tractor sounds like a well cared for gem. I don't know your exact fitting. Is it a female hose fitting on a male cylinder fitting? If so, a hydraulic bolt cutter/nut cracker may be the tool of choice.
Heat? Heat, PBlaster, and time? I know some folks who swear by candle wax on a hot fitting.
At some point, it may be easier to take the cylinder off, take it to a machine shop to drill out the offending adapter fitting and thread in a new fitting.
Often, I get to the page where I make peace with the fact that the problem fitting or bolt is toast and then I feel better about going to town on it, at which point it usually rolls over and gives up. A larger pipe wrench with an extender bar? ...my enforcer wrench is 4' long.
All the best,
Peter