Get down to Harbor Freight (or similar) and get an IR "heat gun" to check the radiator core with. They can look good at the top and be clogged at the bottom. The heat gun will illustrate that quickly. "Creeping heat", at any speed, is usually a radiator issue, from my experiences.
As far as the "high flow" stuff, remember that the head gaskets have flow restriction holes in them to slow things down enough that the water picks up MORE heat from the engine. Chrysler a/c and non-a/c engines had DIFFERENT pulley ratios for the water pump pulley. The a/c pump had fewer vanes than the normal pump, but it was turning a bit faster with those fewer vanes. The under-drive pulley "trick" might be "trick" to have, but I also suspect it can be contributing to the low-speed issues you're having. Just my suspicion.
How much fan noise is coming from your flex fan? Consider that when the flex fan "flexes" and flattens with the higher rpms, it just MIGHT be blocking air flow through the radiator at those road speeds. Whereas a fan clutch with just let the fan turn as the air coming through the radiator will make it turn all by itself.
Just some thoughts,
CBODY67