Just got off the phone with an ASC Indust. representative (they are an aftermarket water pump group) and he told me that GM originally did have a 5 and 6 blade pump but at some point consolidated and went to a 5 blade impeler. They compensated by making these 5 vanes bigger, in order to flow the same amount as the 6 blade. So if you get a NEW aftermarket pump, most likely it's the bigger 5 blade version (at least ASC does it that way, they cast their own). A rebuilt one, well that would depend if it's an early or late (before or after consolidation) pump, unless it's a 6.
