1966 Barracuda radiator shroud / cooling questions

I had mine recored to a 3 row. 273 Commando's came with a 7 blade fan. Your problem sounds like not enough airflow at idle since you are fine on the highway. The best fan at idle i have found is a factory flex fan. I'd look for one off a later slant six since it should be closer it the right outside diameter. The clutch fan takes less power and cools much better than that 4 blade fan, like C130 Chief advised above, but the flex fan pulls more air at idle. Which ever you can find, make sure you check clearances to the radiator and all around before you fire it up.