I don't know for sure about 1956, but the early clarks (pre 1953) had a different bolt pattern compared to the sm420 and the bellhousing was more bulbous to accomadate a larger clutch. What size flywheel do you have? Try Boyce Equipment (you'll have to do a search for the phone number or website)who has rebuilt clarks and may have a bellhousing for you.
jason
