Screaming belt after switching out the alternator

Greeting A-bodies,

So yesterday my new alternator came in. After switching it out I put on a belt I got at a junk yard "it seemed to be in better condition."
I fired her up, and wham it was screaming like no tomorrow. The engine was running, the fan was spinning, but the belt just stayed in place screaming and smelling like burning rubber.

Ok "wrong belt" I thought. So i put my old one back on, and of course she started right up :tonqe:
She was running but the fan was "hit and miss" it would spin like normal, then slow down or stop completely.

Any help?