I am a bit confused from your description of your pump. The following are the two descriptions of the driveshafts that were used in Saginaw power steering pumps.
1. All Saginaw pumps before 1975 were held onto the pump driveshaft by a nut. The driveshaft had a MALE thread (9/16-18 UNF) on the very end. The driveshaft itself was 0.62 diameter and also had a keyway for a woodruff key (1/8 x 1/2) that engaged a slot in the pulley as well.
2. All Saginaw pumps (starting in 1975 and onward) were converted to an interference fit driveshaft and pulley. The driveshaft was smooth, 0.75 inch OD, and had a FEMALE thread (3/8-16 UNC) tapped into the end. You must use a special tool to remove the pulley and you must use another special tool to install the pulley. The special tools will have a screw that threads into the end of the driveshaft and will push and pull the pulley by reacting directly against the driveshaft itself.
You MUST NOT try and just press the pulley onto the driveshaft. By reacting against the pump housing, you will break parts inside the pump. You will also note that the pulley has a machined groove around the hub. The special removal tool will have a flange that grabs the groove to remove the pulley.
These special tools can usually be borrowed from local automotive stores to get the job done.