I believe the top blocks are size 3 - Harken also do a nice ball bearing version but you need to park the keel with a pin supporting it to prevent deforming the balls.
My keel fitted blocks are 1/ an original Maine - tufnol and 2/ home-made to the same pattern as the Maine using a nylon wheel bought from the local chandlery. I am sure yours would clean up and be perfectly usable. I cannot emphasise enough that the main sources of friction in my set up were:
Oversize keel rope
Rope binding against the holes in the keel box top
Poor alignment of rope into organiser blocks on the route back to the clutch
Friction caused on the winch drum as the rope 'slides' up the drum to the self-tail mechanism
These are the system sources - of course on top of that there is the possibility of mud in the keel box!
I believe the original spec weight for the keel was 'approx 350lbs'
BW
Chris