Manual to power steering conversion?

[berghandler]

My grandson and I are trying to convert a 1972 C10 2WD from manual steering to power steering. I have a 1969 C10 with power steering on it and am using it for a blueprint. I have removed the single sheave pulley from the crank end on the 1972 C10 and trying to find a 3 sheave pulley to replace with. I have found a couple aftermarket pulleys but the bolt mounting circle pattern is not the same. Our single sheave pulley had a 3 1/4" BC and the aftermarket pulleys have a 2 3/4" BC. Any ideas why. I have tried at least half a dozen online parts houses but no luck. I'm not sure if the water pump is a low profile or high profile water pump but apparently it matters also, so I need to know how to recognize that to be sure I get correct crank pulley for this power steering conversion.

Thanks