I have to say I don't think it is the rad.
If it works at idle, then higher speeds don't generally reduce cooling.
Higher speed warm up is going to be caused by something else.
blocking air flow would be the first thing I would look at.
for instance a mechanical fan clutched, might be having issues with the clutch. I had one going bad, that allowed the car to warm up quite a bit on the highway.
Or an electric fan, aftermarket, that doesn't allow enough airflow at speed.
Or not enough water flow.
For instance that sucking shut the lower rad hose.
Or a pump going bad, not allowing enough flow.
I would ask if it cools off just fine once you come off the highway.
If so then I would say you don't actually have a cooling issue with your rad.
I'll politely disagree as at highway speeds one is generating more heat. I had a 69 Coronet R/T (members car) with 22 inch Radiator. Measuring the temperature with a laboratory grade thermocouple probe and digital meter. The car had the same symptoms. I could type out a novel on the things tried and hours lost. There most definitely are other things that can lead to the problem but in my experience it is generally an inefficient core or undersized radiator. The final fix was sending the radiator to from Glen ray. He said the issue lies with the quality of the cores today and their efficiency.
That was about 5 years ago and last I heard it has never gone above 190 degrees PERIOD.
I probably lost over 200 hours on that one. Actually completely rebuilt the motor twice. As I said I could write a novel on the things tried.
This was a stock application and maintaining the stock radiator was required.
As to which is better IE: brass/copper vs aluminum. I do not think anyone can answer that question with any certainty as there are way to may variables involved. fins per inch, How they're attached, thickness of fins/ tubes, number of tubes, size of tubes, staggered tubes or not, and the list goes on
We later had a 65 GTO with the exact same issue. This car came in with a reproduction radiator that was less than a year old and a new motor. Getting a core installed from Glen rays suggested source again cured the problem.
