Em glad to hear the problem is sorted out! I had to go to work and didn't get a chance to follow up. I don't think at this point that you need to do anymore with the thermostat. Overflow tanks are nice as stated by Roger to keep the air out and the tank full. If the original didn't have one, you probably don't need one as Jerry stated, unless you keep finding your losing coolant over time. I'm trying to recall if my 68 Mustang had one, but it's been a long time since I've had that car.