You found the "casting number" rascal - woohoo!
Further search of "engine block" casting number info on the web (referred to at least 6 different sources), indicates your engine block is primarily identified as:
1955 First series 235 (Excluding Powerglide)
Some of the sources were less defined, identifying it just as a '53-'55 235.
From this info, sounds very much like you "may"
have the original engine block, especially considering the info you have on build and registration data. It should also be mentioned that your "head" casting number identified it as a later (1957) 235 head. Evidence of repairs done at some point along the line.
The "odd stamped serial number" could be a number stamped by the "Stepvan" manufacturer. Maybe there are other "Stepvan" owners that have some historical info to help???
Am sure others will chime in with more info or leads as they can.
If nothing else, at least you know exactly what you have to start working on.
