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I have been wanting to do this modification for some time now. SCAT sells a crankshaft for this specific purpose. I was just messing around with a 36 LB block that I acquired with no main caps. I think it is possible to fit 59A main caps to the LB block and line bore the block to the later 59A dimensions. After remachining the bearing notches you could fit 59A bearings and you get a rope seal too. I believe the Merc or 8BA crank O.D. needs machined to fit within the LB's dimensions. Rod journals would need to go to 1.999" to use early rods. I have read that it is possible to use 8BA rods with insert bearings and install them from the bottom, so that may be an option.
I'm not sure if the power increase of adding a 4" merc crank would be worth the investment of the machine work but it would be a fun project. Why stop there.....wet sleeve for 3-3/16" bore?