In another post I had the same issues. I have a 408, High stall convertor, Pro Form electric water pump, fan shroud, and 3 electric fans. I could keep the car cool for a while, but then the temp would start creeping up. Read another post about air in the system, and decided it was cheap and easy to try that. I have a few holes drilled in the t-stat, so I lifted the front of the car as high as I could on jack stands and then ran the electric water pump. I saw bubbles coming out for quite a while, and I just kept topping it off.
Since then I can get the temp up and it never goes past 190. I have a 180 degree stat in also....
Give it a try, won't cost you anything.
