I had a similar situation with my BB Coronet, so maybe sharing my experience will help. I originally had a 26 inch 3 row radiator and at hwy speeds in the summer (temps in the 90's), the coolant temp would slowly increase the faster I went. To me, that indicated insufficient cooling capacity since airflow is not an issue at those speeds.

So I decided to try a 2 row aluminum radiator. Same core size, 26 inch with 2 rows of 1 inch tubes (think the brass radiator 3 rows of 1/2 inch tubes). On paper, the aluminum radiator has a larger tube surface area which should increase the cooling capacity. After installing the radiator and driving on the hwy at similar temps, the coolant temp was lower than before. With the brass radiator, the coolant temp was about 200-205F and with the aluminum radiator, the temp dropped down to around 190-195F.

Hope this helps. But be aware that your results may vary.