I concur with Graham.
An imperial propshaft will almost certainly have a BA thread
I would change it for a 5mm shaft
. You may have to change the prop as well, or drill and tap to 5mm. To ream out the bushes you may find it easier to keep them 'in line' by knocking them out, use the old propshaft, and ream / drill both together. A small lathe
When refitting the bushes use the new 5mm shaft
to help keep them centred.
If push comes to shove and you do have to remove the tube a sharp tap on the end with a light tack hammer while gently twisting with Mole Grips usually breaks them loose without wrecking the hull.
Worked on my Sea Scout, same problem; rusted 3/16" shaft and worn bearings🤔
BUT I cheated🤗, I made a new 4mm shaft and bought 4mm I/D sintered phosphor-bronze bushes and Raboesch brass prop to suit😁
Hope this helps more than confuses, Cheers, Doug 😎
PS I agree warming the tube to soften the glue helps. I tried it with a 50W iron on my PTB but it couldn't cut the mustard. So I used a small, kitchen style, gas torch.
Tap tap twist and out they came. Only had to remove them cos a complete hull renovation was necessary🤔 I blogged it on this site, somewhere!!
Now it's cookin' time. C youse guys later👍