Matt:
100c is 212F so at low speeds it is "Warm" however once you get moving and get airflow through the radiator, it drops to 40C about 158F, so it seems as though it is functioning correctly. If you can get more airflow at lower speeds through a more efficient fan, perhaps one that moves more air it would probably help reduce temps at low speeds and idle. The high flow or "balanced" thermostats are made by a company by the name of Robert Shaw. They are brass and can be had through better parts houses. See if you can determine what fan you have and CFM it is rated for and act accordingly. You might also look at your radiator sizing, but I think the fan is the key in your application. Radiator caps are available through the Mustang vendors, 13lbs or 16lbs being the most common.
Good luck
Rick
http://www.stewartcomponents.net/Mer...ory_Code=Therm