Slant Six overheat at cruise speed, especially going uphill

I would say a plugged cooling system (make sure water is actually flowing through the radiator by pulling off the cap while its running only when it's cold. Could also be a stuck thermostat (try putting a 180 in), or a bad water pump, or worst case scenario the water jackets are all plugged up. Shroud will help but like you noted, only while sitting still. Best of luck, Dan