So when it gets hot, how long does it take to come back down to a normal temp if you pull off and sit with it running?
The age-old formula has always been, if it overheats at speed, there's not enough cooling capacity. If it overheats while sitting, not enough airflow capacity.
My feeling is that you simply don't have enough radiator. Remember that most every Mopar with AC used a 26" wide radiator. You may have to figure a way to get a thicker core in there.