No difference in early pumps for the '63 389 and 326. The 4 bolt Water Pump p/n 540069 serviced all Pontiac V8s and the '63 Tempest 336 CID V8 (or 326 if you prefer). I believe this pump was first used for the '61 Model Year.
I don't know the pre-'61 p/n, but 540069 was the Service Replacement for ALL '55-63 4 bolt Pontiac V8 Pump Applications (the '61-'62 Buick V8 that was offered for the Tempest doesn't count). I also do not know if this is the p/n that would appear on the casting or if it is the Service p/n assigned to a pump & gasket package for instance and not the p/n on the pump body itself.
Either way, a 4 bolt pump that works for a '63 389 is correct for you.
For the later model 8 & 11 bolt pumps, most guys have better experiences with the O.E. type cast impellers rather than the aftermarket type stamped steel impellers.
Take Keith's advice, just make sure the cast impeller vanes are the tall ones as seen in his picture.
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