[tsubaki]

And have you removed the bolt beneath the steering torque plate (trim tab)?

We have had instances where the shaft was seized to the crank or the o-ring on the shaft just held it unbelievability.
We'd first engage the foot in gear and try to push down with our foot on the edge of the prop to see if it'll drop. This simply adds a little pressure to the splines and some downward force.
If that didn't produce some movement, we'd then start a wedge and drive system to start separating the foot from the midsection. This starts out as small wood wedges in various locations to protect the foot and midsection and driving a screwdriver between them. Then progressing to larger wedges and chisels as it separates.
Have had an instance where we made no appreciable progress and let it set for a few days with the tension of the wedges. Checking it a few time a day and driving the wedges tighter. Finally easily breaking free.