Please tell me how your car over heats.... Does it overheat when it's idling or when you are driving? Or both?
Key components that control your cooling system:
1) Radiator Cap
2) Thermostat
3) Radiator
4) Overflow Bottle
5) Mechanical Clutch Fan
6) Water Pump
7) Hoses
Now depending on where your car is leaking, anyone of these parts (among the others) are the ones that can lead to overheating. You said that you replaced the T-fitting. Are all your hoses on securely?
I would also like to know what / if any mods you have.
Things that come to my head:
1) you water pump has failed you
2) your thermostat is failed closed
3) your radiator cap can no longer hold pressure
(Below is what I would do)
First, take the car to Autozone and have the cap pressure tested. The cap needs to hold 16PSI. If the cap fails, replace it and try your car again. They can also apply pressure to your cooling system to help you look for leaks..
Next, (If the cap doesn't fix it) Buy a new Thermostat and gasket and replace the one that is in your car. Again, test your car (in your drive way if possible)
If it's still overheating, let the car cool down and restart it... Look at the fan to verify that it turns and that you can feel coolant passing through the hoses. At this point you should replace the Waterpump and the fan clutch.