Need help badly with charging system

The screech was the alternator working as hard as it could, making the belt squeal.
Disconnecting the battery and getting the connections tight on the back of the ammeter was the right step.
Check the belt for being too loose or glazed.
I think you are going to be OK, you just have a "funny" connection.
Next step would be cleaning firewall bulkhead connectors with fine wire brush.