Slant Six overheat at cruise speed, especially going uphill

I agree with the lack of flow theories here. It could be rust sludge in the water jackets or rust chips clogging up the small passages going up into the head, through the head gasket. Also could be the radiator passages need to be "rodded out" or just have it rebuilt. I believe I've heard people mention that cheap thermostats down flow enough when open because the hole isn't big enough.....but that's usually in a performance V8 though.