Through at least 1957, the 18" blade was the standard blade, at least for 1/2-1 ton trucks (the designs between 37-57 might be slightly different but the 18" diameter did not change).
The engine serial number is pressed into the ledge to the rear (left) of where the distributor goes into the block.
The format is:
1234567F54xy
It could have F54 or T54
The xy could be a variety of one or two character combinations.
In any case, the "54" (or other number) is a year designator.
1955 was a transition year, as stated above. After the transition in 1955 the water pump is lower. Nonetheless, if the radiator geometry is as far off as you describe, call the vendor (and let us know what response you get).
