Slant Six overheat at cruise speed, especially going uphill

Hi

It was about 90F today and took the Dart for a trip outta town. After about 25miles of driving the temp started climbing, first to about 2/3 then when it reached 3/4 mark while on a hilly section, i stopped at a rest area and let it cool down.

What can cause this? Ignition timing not set properly?

I understand if its overheating while stuck in traffic but going 70mph on the highway?

Would radiator shroud help?

Thanks

Syed