The 1954 and earlier 216 and 235 engines used a much shorter waterpump. The longer high volume pump was used on the 1955 and newer 235 and 261 motors. They did not however inerchange.
The early 216 and 235 design pulled coolant out of the engine block through two quarter size holes into an exterior pump, then forced it through the lower radiator hose and back into the engine block .
The new style water pump had a propeller that was actually inside a 4” hole in the front of the block. It could move a higher volume of coolant .through the block.
With General Motors infinite wisdom, they designed their new high pressure 235 and 261 engine to easily fit in the place of a failing earlier 216 or 235 engine. The main problem with this engine exchange was the longer length of the new water pump shaft. This necessitated the use of a conversion late style pump with a shorter shaft.