Unless somebody has repositioned the radiator to the front of the mounting cradle, a long shaft pump generally puts the fan into the radiator. Any late model water pump can become a "short shaft" by pressing the fan hub back and cutting the shaft to the proper length. If a long shaft pump fits your 51 truck, somebody must have moved the radiator forward. A simpler fix that keeps the fan in the right position vertically is Dave's adapter plate that allows a 216-style pump to be used on the 55-up engines.
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!