Found this on evilbay. Looks like you’ll need an alternator and dual groove pulley.
https://www.ebay.com/itm/10071-AIR-CONDITIONING-ALTERNATOR-BRACKETS-FOR-1955-59-248-270-302-GMC-6-CYL/222808573940?_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D2%26asc%3D49917%26meid%3D6777fadf612040009fa04b0e7192b594%26pid%3D100623%26rk%3D1%26rkt%3D3%26sd%3D322569067118%26itm%3D222808573940&_trksid=p2047675.c100623.m-1
brackets