http://www.sccoa.com/forums/showthread.php?t=28797&highlight=power+steering+pulley
The link above is to a group purchase on a CNC machined aluminum two piece pulley. The stock pulley is about 2lbs, this one comes in slightly under 2lbs according to the posts. It's designed to have a hub that is pressed onto the p/s pump shaft with the pulley fastned by bolts.
I am not aware of any plastic replacement. There is a manual gear rack out there for mustangs that will fit in the SC if you want to get into some real weight savings.