Did it overheat before the radiator was done with a new core. I had the same issue and pulled my hair out trying to find it. It was Air flow. At that speed you don't need a fan.
My radiator tubes were bulged. Could have been from being over pressurized Possibly from the shop that installed the AN fittings or from freezing. I had no air flow . Hold a light behind the rad and look through. Or hold paper in front while its running. 5000 stall, 456 gears, never went above 180 cruising a 60 mph. after installing a new radiator.
