I have a 68 chevelle ss with a fresh rebuilt 396 425 hp. I've only driven it a couple of times. I have the windstar electric fan setup and a 180 degree stat.
The sender is located in the driver side head. While driving at 45 mph, my temp would creep up to 210/212 degrees. When i slow down and come to an idle, it drops back down to 180-190. What could cause it to be that much hotter while driving?

A few things to think about here .

the windstar fan set up may be restricting air flow at higher speeds as theres a full shroud with just the fan holes to pass air trough ... cut some holes in the blocking areas and fab up some rubber flaps that close when the fans come on and open to let it breath at cruse speed .

another thing is coolent flow ... a hi flow water pump ... three things that ad cooling cfm ...flow .. and a efficient rad for h/p and btu out put of the engine ... as your enging is fresh and tight it will run warm till broke in ...the fact that the sensor is in the head may add 5 to 7 degrees to temp gage .. a true reading would be to get a I/R gun and shoot the t/stat housing to see how close the gage it to real readings .
try a 70% distilled water to 30% anti freeze mix for better cooling .
Don