Thanks all for the comments, indeed this truck was restored in Dallas area, which is perhaps why the electric fan was used. My area is also typically in the mid 90s in the summer, but less slow traffic... but the controller mentioned by Fibonachu seems the right answer for me. I found a TorqFlo thermostat/switch at Auto Zone (733653) that seems ideal for my application, and it was a little cheaper than the Flex-A-Lite referenced (about $16). (see picture of contents).
The TorqFlo has a unique attaching scheme it seems, it uses like a Ty-Wrap type plastic "wire" that you push thru the radiator (very small) and then attach a clamp and the temp sensor to it, rather than trying to wedge the fairly large temp tube into the radiator itself like Flex-A-Lite suggests (would not work for me). My radiator though has the back side closed to enhance air pulling effect, so mounting the sensor in front isn't ideal it seems. I am rather thinking though about mounting it to the top back side of the radiator where the hose connects, using thermally conducting epoxy to provide a consistent temp reading.
