1999 Chrysler 300 leaking coolant

"What to try"? Why don't you diagnose the problem first? Throwing random parts at a problem is by far the most expensive and least effective way of solving a problem. You'll spend less money getting a professional involved.

What is "fine", the thermostat housing or the middle of the engine? What is the "middle of the engine"? That can mean five different places.

Which engine do you have? Does the radiator fan turn on? How hot is the air coming from the heater? Circulation problems will result in overheating and cool air from the heater. Radiator-related problems result in overheating and very hot air from the heater.

As for the leak, if you can follow the coolant up to the source, you just need someone to identify the part and the repair procedure. If it's too slow to find you can add a small bottle of dark purple dye to the coolant, drive a while, then search with a black light. The dye will show up as a bright yellow stain that you can follow back to the source.