I would look for a small town-to-town 2 lane with minimal to no stop signs and minimal to no traffic to test on.

A 45 or 50mph kind of a road should do it.

Pick out some places to stop and work on the car or let it cool if you have to.

Drive there. Then take the fan and shroud assembly off and put it in the trunk. Then test. Obviously abort/stop if there's a problem.

If it seems to prove the idea the shroud is blocking airflow, then you have choices.
Try to create flaps (rubber, plastic, other) for air to travel through your existing shroud at speed that the fans can suck closed at low speeds, or try to shroud just one fan,
or go back to the drawing board.

When I ran the HHR fan on my GTX I tried a similar temp switch to activate it, and had a similar high speed issue.
I realized the time I could potentially put into getting flaps and so on set up to my liking, and wasn't into doing that.
Eventually I sold all that and went back to a belt driven fan.
I'm sure you can get it to work if you spend enough time playing with it.