Never store your car for years with old coolant or tap water,
This is what happens
better to have nothing in the engine

If the water pump is that bad, I'd hate to see the rest. Did you pull the intake so you can check the top block and intake passages? I don't envy you on this one!

Wow. That makes three like that lately on this site alone. I stored my 455 for over 30 years but I made sure to drain it. I guess 'ran when parked' is moot at this point.

I flushed the engine and radiator out, should be fine. This is mainly a reaction with the aluminum timing cover and aluminum water pump. I already had this engine running with the belts and water pump pulley off. Runs fine, new pump is already on.

Are you old enough to remember when Prestone sold a two-part can of cooling system cleaner? One section had the very aggressive cleaner, the other section had the neutralizer. That stuff would scour your cooling system like it was steam cleaned. I haven't seen it in years. What I use now is Cascade dish washer soap. It will make the cast iron look like new, but will attack aluminum, so you can't leave it in too long. Baking soda will neutralize it, then lots of fresh water. Both the Prestone and Cascade will find and expose any leaks so be prepared for that.