To answer everyone's questions:
I hadn't thought of an airflow blockage. I'm running stock bumper and valance. @Peter A
the car is in the same exterior setup as when you had it, and I did change the thermostat (granted, it wasn't a new one that I put on, just a good used one on the shelf) with no change. I do have a brand new one from Centerline that I could try. I like the compressed air idea that @cda951
mentioned - I'm going to try it, too. Maybe something as simple as that is the issue.
It's incredible, but the York compressor still works, and the A/C (Tropic Air) still blows quite cold. The other day, I actually had to turn it down...
Before the engine rebuild, I had the stock twin fans. They never came on while cruising on the highway, and the temperature stayed at about 180 no matter how hard I pushed, nor how hot it was. Stop and go in the summer was always an issue, but as soon as I started moving everything was fine. I previously had a Milano fan on it and it behaved the same before and after rebuild.
Next on the list, I suppose, is 1) check the timing (too advanced?) 2) thermostat 3) cooling system blockage (either air or in the hoses). If all these three check out, then it has to be the ANSA headers on the car. But if that's the case, it's still is perplexing. They are meant to improve performance when you're pushing hard - it's a big irony if they preclude you from pushing hard because they cause you to potentially overheat. *
*I never have actually gone into full overheat, but I've also not run the car on the highway for more than 40 minutes at a time, nor have I pushed it that hard.
**I did go from NYC to Allentown in winter, but it was a 17F degree day and the car struggled to keep 180F.