With the disassembly complete I was surprised to find two sizes of water pump impellers. One is 3 1/2" diameter, the other is 4 1/2" diameter. Both appear to be OEM. The one from Dead Nuts On is stamped steel - here are Jack's comments:
This is a service stamped steel water pump impeller for the following Ford engines:
352, 360, 390, 410, 427, 428, 428 PI, 428 CJ, 428 SCJ (FE Series engines)
* Ford originally used a larger cast iron impeller (not available) that was more efficient than this generic stamped steel impeller at high RPM's.