I have a 55 gmc and I believe that a 270 engine WAS an option for the year 1955. reason being is that whomever first titled the truck, (a local lumberyard I think) they titled the truck using the engine number. and I still have that engine in my truck.
Whether a 270 was offered or not, the proper procedure when changing an engine in a truck titled by engine number is to get the title changed. That could be what happened in your case.
