It is quite possible you have the differential, housing and springs from a '55 first. The '55 1st rear end is a popular way to convert to open driveline because they will bolt right in in place of the torque tube assembly.
That engine casting number is a mystery though. I have a 248 with a 1955 date code with that casting number. I at first thought yours might have been a dealer replacement and the old titling serial number stamped on it. GMC replacement engines came with a blank serial number but the displacement was stamped on them. It's strange that one has 228 on it.
