I don't think the water pump rpm had anything to do with the welds breaking--it was a marginal design change for a non-existant reason---and an inferior method of manufacture.
And then there was the mix/matching of non compatable pieces---Ford shortened the water pump to allow the 302 to be put in the t bird/cougar late 80s or early 90 as the engine was too close to the front of the car for federal crash specs---there was a complete series of parts involved with everything that had to do with the accessory drive / mount system. Your mounting and modifying( what rodders do) of these components ( shortening dampner drive hub) might of had more to do with setting up a vibration/harmonic in the drive system that caused an faulty weld to fail.