Here's what I'm planning to do, since I don't have the spline tool, or access to it ('97 AAZ w/AC).
My alt has a T-50 recess in the shaft, so I plan to use my 3/8" impact tool to loosen/remove the retaining nut (turn shaft CW, while holding nut). Note: measure offset of original pulley, to ensure the replacement fits properly. It might even be wise to 'clearance' the shaft to start with. Fit new pulley hand tight and verify offset. Use a 'strap wrench' as if trying to turn pulley CW ( the way it drives), set impact tool (or drill/driver) for CCW, (moderate torque), and tighten. This should tighten the pulley sufficiently to keep it in place. The spline tool will be required, should you have to remove the pulley. You might want to check with your local dealer or alternator rebuilder, to see if they have the spline tool, in case you have to remove the pulley to clearance the shaft, as noted by Libbybapa.