I have to wonder how congested it is under your hood. For the cooling system to function, air that passes through the rad has to be able to exit from under the hood. Many guys close up most of the air exit routes with street rods. When the flathead 6 was there, air exited around the sides of the engine, through the louvers on the inner fender panels, and under the floor around the bell housing. When you fill those areas with V8 engine, headers, brake boosters, and remove the louvered inner panels, you leave little room for the heated are to exit.Look for ways to improve air flow through the engine compartment.

Also, over heating on the highway can be caused by a lean engine, too advanced timing, or moving the coolant through the system too quickly. Could be a defective gauge but really, 210 is not overheating, water boils at 212 and if its under pressure like in a closed cooling system, that boiling point can be as high as 250. It may be higher then you (and I) would like, but not hot enough to damage the engine. How does it do when it gets hot outside, or driven around town? Gene