How do you remove the water pump from the fan when the water pump has a pressed on pulley?

In your situation, can't you unbolt the fan from the water pump flange/pulley and bolt it on the replacement? Such as:

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The fan has been removed attached to the old water pump and pulley set up. Is there a part that allows you to hold the pulley to unbolt the fan? So far they have been unsuccessful in releasing the bolt on the fan.

I've had some experience with these... are you working on a gas engine? Some of the pulleys on the Rams have 4 small holes in the front of the pulley. I have stuck a small screwdriver through one of the holes and wedged it against the body of the waterpump, and used a large crescent wrench to remove the fan/clutch assembly from the water pump shaft.
Some Ram water pump pulleys don't have any holes, and a strap wrench around the pulley may be the only way to hold the pulley to get it apart.

I just replaced mine 2003 4.7 you have to cut off the shaft of the pump
[ Back side] put the shaft in a vice tourch the large nut it will release then you can unthreed it