Until you add the shroud, which WILL help (it did on my 57 Chevy Wagon), consider adding a surfactant. I have used Redline Water Wetter in the past.
You may (or may not) know what a surfactant does. If you don't - a surfactant is a chemical that reduces the surface tension of water. Water has high surface tension (that is what creates large bubbles you see in the bottom of a deep pan of water when it boils). If you watch the boiling process, you’ll see that these bubbles remain on the bottom of the pan for a moment before breaking loose and rising to the surface of the water. Those large bubbles are created and held at the bottom by water’s high surface tension. I believe that large bubbles also occur also on the water jacket side of the combustion chamber when your engine is making power. When those bubbles are created, they reduce the heat transfer characteristics of the water and form hot spots. A surfactant reduces the water’s surface tension and allows better heat transfer into the coolant at the head, and it is more efficiently transferred to the radiator cooling tubes. The result should be a lower overall operating temperature.
I saw a reduction in temperature in the 57 Chevy, although I did later replace the electric fans with a flex fan and a shroud.
IIRC, the Water Wetter was about $10, and it won't hurt to try it as an interim step.
I do not recall ever having this issue with my 200 inline six, but my Infinity G35 is known to have an issue "burping" all of the air out of the cooling system (something I recently discovered after the thermostat and lower radiator hose were replaced). For that engine, there is a specific process to rid the system of air. And, if you don't, the temperature rises an astonishing amount. It might be worthwhile (and free) to make sure your car is at an incline with the radiator cap being at the highest point, run and start the car with the cap off and the heater motor on high, then add water if needed and install the cap.
Good luck.
BTW - the most optimal cooling system I ever was in a 55 Chevy Nomad. As a kid in 1969 while camping in Yosemite with my parents, we met our camping neighbor. He used his 55 Nomad to tow a travel trailer. He owned a radiator shop and ran two radiator run in series, one ahead of the core support and another behind it. He said he never had overheating issues. I believed him, my Dad did, too.
