Casting numbers are the best way, and there are numerous number decoder websites. 54 and early 55 engines run the same water pump as a 216, mounted high on the block just below the thermostat housing. Later 235's and 261's have the water pump mounted about 3 inches lower down, and a big hole in the front of the block for the pump impeller to run inside the water jacket. "Pre-68 Dave" who is a longtime member of the stovebolt site makes and sells an adapter plate to allow the early pump to be mounted on a late-design engine. One of our resident number crunchers will be along shortly to provide all the casting numbers you can stand to read!
The pump position is important, because the fan position won't be correct if a late model engine is installed in an early chassis. Mid-year 55, the hood line was lowered and the pump was moved down so the fan would clear the hood.
Jerry
