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David, yes, the 8BA crank will fit the 42 block, you can use the 59ab aluminum upper rear seal retainer, most flathead places has the special aluminum rear seal retainer for the main cap, use the 42 main bearings, I would use the 49-53 rod bearing and rods. You can use the 4in. Merc crank also, but grind a little notch about an in. long and about 1/16 deep on the bottom of each cylinder so the crank rod nut wont hit the bottom of the cyl. Nothing to it. Walt