If the rubber is hard and glazed, it's time for a new belt. The belt only needs to be tight enough to not slip on the fan and generator when you goose the throttle.
Remove the fan belt and see if you still have the squeek. Hopefully you don't have a bad piston from the overheating. If you sill have the squeek with the belt removed, I'm betting a piston is galled and has rings tight in the grooves. Usually #4 is the first to go.
maybe the radiator boiled over and the antifreeze got to the belt, this is one problem that makes the belt make noise, try washing the belt off, or the belt dressing nuetruelized it, ...just my 2cents
john