Does the alt belt drive the water pump too? Could be a worn water pump pulley (or a worn crankshaft pulley for that matter) that is causing the problem. Start it, rev it a couple of times, then shut it off and make sure which pulley is slipping by which pulley is hot. If your trying to fix the wrong pulley, it will never be fixed.

The Jeep I had had a bad crank pulley, it was down where all the road salt attacked it for years. Put a different crankshaft pulley on it and the squealing went away. Gene