Two hammers of equal weight tapped simultaneously on either side of a seized shaft will often free it. The problem here is the space is so restricted it is almost impossible to get any leverage or heat where you need it.
If you keep lifting and dropping it you will wind up damaging the hull.
The Amel splitter has to be your next best option with as much PB Blaster, diesel and paraffin, white vinegar and anything thing else you can think of poured on to it.
If that doesn't work, you can remove the entire assembly from inside the boat when you are hauled out. Trust me, that is definitely your final option.
I have seen one of those shafts heated to white hot on a bench and it still wouldn't come apart.
Sometimes there is an extra grub screw (i.e. five instead of four) glassed into the bearing housing on the foot of the shaft, this is not relevant to your current problem, but might come into play later on when you get that far.