I am beginning to suspect that part of the highway speed cooling 'equation' is the engine compartment size and subsequent air flow. We are dealing with a smaller compartment than your later Duster. There is nothing especially constrictive in the engine compartment: 340 small block, AL heads, headers. In front of the 22" rad is a full width AC condensor, and a half width trans cooler. So there is some flow restriction in front. The hood to core support seal is in place. HP production at interstate cruise should not be too much; there is nothing unusually 'un-aerodynamic' about the car....well, besides being an early A body!
So it all comes down to airflow and coolant flow and heat transfer. Coolant flow is (hopefully) better with a Milodon high flow pump. But that can only do so much if there is a flow restriction elsewhere.
