AFAIK, the only differences between the early water pumps and the later are that 1) the later have one flush head screw, and 2) the pulleys are different. Neither is a show-stopper; you should be able to source a later type pump and swap out the pulleys.
I also pointed out in the Book that the seal is available at a Grainger’s as it is a industrial standard. In fact, you might be able to get a better one – Viton goodies and all.
There are impellers from Chevy V8’s that will fit and work, even though the vanes look completely different. I suspect the Chevy impellers work better at idle. You want the ID to be the same or larger, never smaller.
The bearing is standard water pump fare, virtually every automotive water pump has a similar bearing. What makes it unique is the press-fit sizes on both ends, one to fit the impeller and the other to fit the pulley. I have long hoped that someone would figure out exactly which car out there uses a water pump bearing with exactly the same ends on the shaft. If we could find that, you could go into an AutoZone, pick up a rebuilt pump, rip the brand-new bearing out of it, toss your old bearing back in the box with the pump, and return it for the core charge.
If it were me, I would insist upon the gortex sandwich gasket in between the pump and the cover. Most rebuild shops apparently send them out with paper gaskets, and you can’t change the gasket without pushing the impeller off the shaft and then back on when you’re done.