Pressed on. You may get away with re-installing it using a bearing race permanent loctite, but more likely you should rebuild the pump. It all depends on if the wear or out of spec is on the shaft or in the pulley bore.
A kit (new shaft and bearing) is only about $30 from MAC's.
http://macsautoparts.com/early-ford-...R3CHL1141868E/
If you are going to re-install the original pulley, you cannot merely tap it back on in place, or you may damage the bearing or shift the bearing and have the seal leak. The pulley must be pressed on to the pulley by pressing on the shaft at the impeller. You could also press the pulley onto the shaft, but you would have to support the shaft at the impeller, rather than pressing the weight onto the housing.