The "low water pump" engines were needed in midyear 55 so the fan would clear the lower hood profile when the body style changed. If you try to put an early 55 engine in a 55 2nd. series truck, the fan would hit the hood.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!