Every thermocouple has a time constant... some are faster than others. I experimented with EGT probes on a turbo car and found that longevity was inversely proportional to time constant (due to rugged construction) But you're right, for the absolute most accurate and repeatable temp measurements that's the way to go. I assumed (there's that word again) that mine is close since the t-stat opening temp (on 2 different 180 stats) is very close to what I see on the gauge...

I just find it hard to believe that a couple of pieces of cardboard baffling are going to make a significant difference to the airflow through the radiator while driving with so much frontal area open
I don't have an overheating problem at idle either, just wanted to drop 10 degrees while cruising.

If the core is that marginal I need a better one anyway. I do agree that (sometimes) one gets what one pays for Glen-Ray is a bit steep for my budget but I'll buy one if I have to!

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. twocents beer