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The shaft on the car version is a 5/8. The truck is a 3/4 and it's a bit longer.
http://www.theautopartsshop.com/search/default.aspx?year=1960&make=ford&model=f-100 pickup&submodel=Base&engine=8 Cyl.4.8L&parts=engine water pump
This is what I've discovered so far. Good prices as well, but the 3/4 is out of stock. I ordered a repair kit from Dennis Carpenter.

Some big trucks had a high mounted fan that bolted to a vertical boss on the pump casting, they also had 3/4" shafts instead of the car/Bird/light truck 5/8 shaft. That may be what this pump is.

John - "The Hoosier Hurricane"

I didn't know there's a difference between car and truck water pumps. So I have a reman Cardone car pump for sale. I'm saying it's used because it's been mounted. I can include new gaskets for it. I paid 80 bucks for it. Give me $40, shipping included, and it's yours