As stated above it could be a head gasket on backwards or on the wrong side. Ford 5.0 L engines would overheat very quickly if you put the gaskets on wrong after a head swap as the gasket plugged a coolant passage. I had a bad fan clutch once my Mustang 5.0 but it only had a problem while idling, not while driving as the airflow from driving was enough to cool the radiator. If you had a shop do the head gaskets and it started ever since they did the job, take it back and mention it to them and have them take a look since they did the work.
