I live in Florida and my 55 Tbird ran hot in traffic.  A new thermostat will most likely not fix this issue.  You can put different fans on it, dump special coolant in the radiator and call a witch doctor and you will still have overheating in the Florida sun.  The way to fix this permanently is to upgrade to a Champion three row aluminum radiator and an electric fan.  This will fix your overheating forever.  I did this whole swap for around $350 and it was worth every dime.