Water pump packing nut removal

[1965 Mustang]

How do you get the water pump packing nut to move forward to install packing? With the water pump installed on the engine the nut just spins and spins on the shaft but will not move forward no matter how long I move the nut counter clockwise. The shaft remains stationary. if I removed the whole water pump I could probably figure it out, but I don't want to remove the radiator again. Thanks