first of all, i believe the only pump with the 4" hub is on the '69 engine, the rest are 4.5"....i believe.
as for measuring, if the water pump is off of the car, start the measurement at the machined surface on the impeller side of the housing where the gasket would go and measure to flange surface where the fan would mount. for ease of measurement, use a straight edge and line it up on the flange and measure back to the housing.
hope this helps.
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 "Second place is the first loser"
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