I fit a Ram Cummins rad, CAC a GM AC condenser and a medium truck trans cooler behind the dual headlight header panel/grille.
Your cooling fan and clutch are tasked with providing airflow through the cooling stack however, if the perimeter of the rad shroud is not sealed and there are air-gaps around everything else in front of the rad then air is drawn around instead of through cooling stack at speeds you mention.
The shroud isn't sealed to the Cummins radiator very well, that could be a lot of the problem as well.