Is the pulley a bolt-on type, or is it pressed onto the pump shaft? I have converted 216 and early 235 water pumps to 2-sheave configuration by machining away the outer part of the pulley with the single belt groove and adapting a 2-sheave pulley for a Ford 302 V8 pump to the bolt pattern of the 216 pump hub. Stovebolt GMC water pumps of any type have been extremely scarce and expensive in the past, but I seem to remember that somebody has started rebuilding them again. For a while, finding a GMC pump was sort of like searching for the Holy Grail!
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!