so to be clear you hear the fans kick on at 60 mph? or see them or whatever?

can you find a place you can coast for a while? see if you shove it into neutral see if it still stays the same temp? say coasting down from 80 or so. Maybe a down hill run.
That would prove if you have enough cooling vs air flow being the issue.

If air flow is the issue, which seems unlikely, you could also try cutting flaps in the large flat area's between fans, top and bottom.
just cut a chunk out of the cowling there and use some plastic sheeting on hinges to flop back down when there isn't enough air flow to push them open.
That will prevent them from bypassing the fan when you are at idle sitting still, but let them open up at 60 mph or so to let more air pass through the rad.