1. Verify what your temp gauge is telling you by shooting the radiator hose where it exits the thermostat housing with an I.R. gun so you know whether the gauge is accurate (most aren't, especially if the sending unit has been replaced). You may not have a problem at all.
2. How old is your radiator? If it's original, it's lost at least 30%-40% of its cooling capacity over the years due to scale accumulation in the tubes; the "flow test" doesn't tell you anything about the radiator's heat rejection capability - only that it isn't plugged. If the radiator has been replaced, is it the equal of the original in actual cooling capacity, or was it bought on "price"?
3. Are the correct fan shroud and fan clutch still in place? They're essential to idle and low-speed traffic cooling, with the tips of the fan blades located half-in/half-out of the rear edge of the shroud, and the shroud should be sealed to the radiator.
4. Put the 180* thermostat back in - it modulates coolant flow and has nothing whatsoever to do with operating temperature; it only determines MINIMUM operating temperature, like in the winter.
5. Check your timing and vacuum advance operation; retarded timing and inoperative vacuum advance are major contributors to heating.
6. Is your lower front air dam still in place? Your Corvette is a "bottom-breather", and will definitely heat up on the highway without the air dam in place to deflect airflow up through the radiator.
7. Are all of your original rubber radiator-to-support seals still in place? If not, that's allowing incoming air to bypass the radiator core instead of being forced through the core to carry off heat from the radiator.
99% of cooling problems are related to not enough radiator cooling capacity, airflow management through the radiator, ignition timing, or combinations of those issues. The original cooling system (especially on small-blocks) was perfectly adequate; attempts to "re-engineer" it usually fail miserably (although they enrich the folks at Jeg's and Summit).
