If you are running an electric fan, there are many options for fan controllers with prices all over the map. Here is an inexpensive adjustable one from Derale that works very well. It has a threaded fitting that replaces your radiator petcock, or better yet, plugs an extra fitting in your radiator. It has instructions for an override switch too. It is super accurate:
http://www.summitracing.com/parts/DER-16749/