Tip it on its side, against a building, or tree, then try this:
Take a propane torch, heat the shaft and surrounding area to about 400 ºF (I assume that you already have the nut off) Then take an IR thermometer, and when the heated area around the splines cools to about 300ºF or so, take a bees wax candle, use it like a welding rod, melt it into the area and let cool down. Repeat the trying to take it off procedures you have tried before.
Most of the cross bars and stump jumpers that I have gotten off, have tapered shafts and splines. Generally, if you have that joint waxed, whacking one side of the blade holder with a sledge hammer will have it jump right off. Note, I haven't seen everything, yet, so there is always room for surprises...