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

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