The 2000 model years didn't come with a cooler in the rad, just a small oil to air cooler, I swapped this one out with a 6.0 OTA cooler and have never seen over 170. As long as you aren't seeing sustained temps over 200 degrees you are okay. If you don't have a aftermarket trans gauge get one!!! The factory units are worthless. The biggest killer to any auto trans is heat so it is very important that you know what the temp is.
Kevin