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

Ok First step first...

Its not the thermostat (opens fine in hot boiling water)

Before


After


Tomorrow I will try flushing the cooling system and fill up with new coolant/water mix and see how it goes.

Syed