Slant Six overheat at cruise speed, especially going uphill

HemiMark said

I know this sounds stupid but are the rad hoses really old...... are they collapsing?

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What Mark said. The lower hose can collapse at speed. (It gets sucked shut.)

A stuck thermostat would overheat it at any speed.

A shroud only helps at low speed. It directs the air through the rad.

If the radiator is old, that's most likely the culprit.