[56markII]

First thing to do is pull the heads! Check for cracks of course and perhaps a piston out of the group of 8 is bottom dead center and measure the stroke. Another thing is after determining you don't have a Humpty Dumpty riddled with cracks regardless of whether or not it's a 255 or 239, soak the valves and make sure they at least move!! I have had to deal with some in the past that were broken loose and turned over with severely seized valves resulting in the hollow non adjustable lifters being collapsed! IMHO a crack free block is worth more than worrying about a 3 3/4 or 4 inch crank. What good is a crank without a block?? As for the bell housing a 49-50 Mercury has an SAE style bell housing like a truck as well.

51 MERC-CT THANK YOU SOOOOOOOOO MUCH FOR THAT CHART!!