Go to YouTube and punch in my name Kirk-NJ and some videos will come up about the Sherman trans and how to shift the different ones.
You can pop the steering box and access thru the top. Go back to pomester's photos and you can see two rails. Spray those with some BP Blaster and both sides of the case top where the shifter attaches too. Try working lever while spraying. It might be enough to free it up.
The Sherman tops had an open rail system and later a close rail system, where the rails are not exposed. I have never seen a closed top rail system on the Fergusons.
I have been able to take the top cover off and remove it thru the steering box hole on the later hundred series Fords but it's a fight coming out and going back in is even worst cause the forks have to line up just right. I don't think you can remove yours from the steering box hole.
Try the spray and a little pressure and you can even tap the exposed rails will a small hammer. And I said tap not wail. If
You can't free it up it might be easier just to split the tractor and pull it out to work on it.
Post back your results.