What was the purposes of the orifices in the radiator inlet? Never fall for the old "the water isn't staying inside the radiator long enough to cool" myth ... that's not how thermodynamics work.
So you have about a 15 F delta/T between inlet and out of the radiator off the freeway ... I wonder what the factor spec for temp drop through the radiator is.
At freeway speeds your delta/T drops to 10 degrees F ... which would indicate to me a lack of air flow (unusual at highway speeds), or some situation with the engine where it is producing an inordinate amount of heat at 3000 rpm.
If it was a coolant flow issue you should see a large delta T inlet/outlet at highway speed as you would have an plenty of air flow, but less than optimal coolant flow.
What type of shroud did you add? Shrouds are good at pulling more air at low mph but the wrong one can hinder air flow at high speeds. The later model factory shrouds had an important taper from radiator to fan opening that did a good job pulling air through the WHOLE radiator rather than the area directly in front of the fan. Fan depth inside the shroud opening is also very important.
Pictures would be a great help. Lots of time experience guys can look at your setup and spot something.
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