My thought is does it have a thermostat in it? If not it needs a restriction ring. This may sound stupid but I parted ways with this car for a while after I built it.
Fan blades not turning (turned off) may be blocking airflow. Also check airflow going around radiator instead of thru it at highway speeds. I just fixed a Corvette with the same problem by sealing the top of the rad to the hood, fender to fender.
