That was really helpful too

Have you actually read any of my posts?

OK, some more data for those who think I'm not actively trying to research the cooling too...

I managed to get the fans out by only draining the coolant and removing the top hose. So I refilled it and went for a ride (78F and humid at the present).

A very strange phenomenon, maybe someone can tell me why: Warmed it up a bit to 160 on the gauge, started driving, temp went to 183 and t-stat opened as expected. Fast drop to 165 as the cold coolant in the radiator hit the engine, then gradually back up to 182. I drove a couple of miles at 30 mph and it stayed on 183. Thought I had my answer for sure (airflow obstruction). But wait, there's more...

So I turned around in the driveway (10 sec. max) and while I was doing that, the temp popped up to 190. Started down the road at 30 mph again... and it would not come down below 188 in the next mile.

Made another 3-pt turn down the road, and it rose to 198. Another mile, would not come down below 196, so I drove back to the garage and shut it down! Went up to around 215 due to accumulated engine heat. I put the garage fan on the radiator and it's dropping slowly (the electric fans would normally kick on and off a few times).

The gauge does not seem to be sticky, so I have to assume for now that the temperature readings are real.

How on earth can it be operating steadily at one temp, but "bump" it a little while idling/stopped and it won't come back down? This is outside my area of experience and I've never seen this on any other car I've driven in the last 40 years!

And I know it's not obstruction of the airflow by the shroud since the radiator core is sitting there wide-open...

by your description you are either not getting enough air through the radiator while moving Or the radiator is too small / Inefficient to keep up with the heat being generated.
You can try some temporary cardboard baffling to trap and force the air through the radiator. Do not forget the seal between the hood and core support as well as any other areas the air can be escaping through. Last thought would be are fans/ shroud blocking the air flow ??? Some electric fan / shroud setups have rubber flaps that are pushed open at higher speeds letting the air flow through rather than acting as a barrier.
A long shot but make sure your lower hose isn't collapsing if it does not have a spring inside it Keep us posted

Thanks. If it turns out I need more radiator, that looks like a good choice without breaking the play budget.

My "no-brand" (it looks just like a Champion) has three rows of smaller tubes and a lot of them.
ECP claims that the two 1" tubes cools "substantially" better than a three-row, but I wish they had some BTU and temperature drop numbers to back it up.

Thanks for the recursive post.
Regarding airflow, the radiator is just sitting there behind the grill, basically open to the incoming air. I recently posted that I completely removed the fan/shroud assembly completely, and still had temps well above the t-stat opening when driving at 30 mph. Would ducting really be necessary or helpful? In your experience, at what speed does the fan stop contributing? Is 30 mph not enough?

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