Stuck in traffic 340 got up to 220

If it's not some kind of combustion leak into the coolant system and or it has a poor tune ..I'd say that your radiator is too small

At hwy speed you don't even need a fan.
Fans are for stop/idle and pre 40 mph cruising. Your water pump and air passing through the radiator do the cooling at hyw speed. 6 or 8 impeller water pump?
Play with more or less timing maybe.
Check that you have sealed the front core support around the radiator and that the fan is within an 1" of it.
Fwiw Should be using around an 18" fan if without fan shroud.