Okay,
First things first.... Are you wanting to remove the fork or are you just wanting to get this shaft freed up so it will pivot properly? These shafts and forks are not that tough... They are just nasty and stuck with rust and brake / clutch disc dust along with what ever else has gotten into the clutch housing.
If you are just wanting to free the shaft so it will pivot, you need to first, clean the housing up as good as you can. Stuff a rag or something in the axle splines and take a pressure washer or a good strong water hose and wash that thing out as good as you can. Get it clean! If you can, blow it out with compressed air.
Then remove the anchor bolt that holds the small lever at the top of the shaft. Then soak the bottom of the shaft with penetrating oil where it pivots in the housing and soak it at the top of the shaft where it pivots as well. Then using a good drift punch, place it on the top end of the shaft and try driving it down with a hammer to break it loose.
There is a chance you will drive it down far enough to knock a flat looking metal plug out of the bottom of the clutch housing. If you do, that's okay, it is a simple concaved freeze plug that is easily replaced.
Once you break the shaft loose, you should be able to start working it with the fork or put the anchor bolt back into the lever at the top of the shaft and hook your steering linkage to it. Then use the steering lever and work it until it frees up. Keep spraying the penetrating oil in as you do and it will free up.
This is going to take some time and patience. Patience being the key word here.
It takes lots of elbow grease! I've done at least one hundred of these and they all turned loose by doing this.
Personally, I take the shafts completely out and then clean and polish all the components and the bushings in the housing. Then coat all the pivot points with "Anti Seize". You'll be good to go! Good Luck!!

40 plus years working on JD 350s, 400Gs, 450s and other equipment both Ag and Construction.