There was a raging debate on Classical Pontiac about replacement water pumps a couple of years ago.

After lots and trial and error by many people over there, the consensus was to avoid the stamped impellers like the plague and adjust the dividing plates for a very small clearence like the discussion here a couple of weeks ago.

There were also comments that NAPA was a good source of the cast impellers but you had to go through many of the same part number to get one. I made sure when I purchased mine from Ames that the one I was going to get was cast. The salesperson pulled one off the shelf to check and then sent me that one.

Some people at CP have even gone so far to have a shop pull the cast impeller off their old pump and install it on a rebuilt one.