You are correct, March or April was the date I have always used as the stop date. The head number is also a later part, from '56 on I've been told. Your truck may have had an engine swap somewhere in it's history. The "848" head has higher compression than the "913" head that was on earlier models. I have a '55 here with a "913" head on it but I also have a '56 here also with a "913". Then I have a '58 motor with an "848" head mounted on it. Go figure and good luck.
