For what it's worth.....
Tune the radio on to a station playing zydeco 24/7, get a cold one..........
Assuming all thread is what I know as studding, continuously threaded round bar, get a larger, much larger size.
Make an end cap for the shaft that will support the studding 'half on' the out board end of the shaft, after welding it to the gib head key.
Squirt plenty of 10% ATF / 90% acetone around the place and grease the thread. put as much tension as you dare on the studding, making sure it stays parallel to the shaft. Keeping it all parallel is the key to gaining maximum force.
With the shaft at 12 o'clock, beat on the boss at 3 and 9 o'clock with 2 similarly weighted hammers, at the same time for a few seconds.
Have a good chug on the can, turn the radio up and repeat every few minutes, whilst doing something else, keep squirting the ATF mix on. Pack up when you've had enough, but leave the tension on.
Try again tomorrow.
The theory is that you will fractionally ovalise the bore, allowing the lube to travel along the shaft. I've had some success with similar methods removing marine propellers and couplings from stainless steel prop shafts.
As Butch said, if there's room try and drive the flywheel towards the crankcase, with the tension on. Above all take your time, more beer and zydeco.......
Cheers Stef