Cooling issue on highway

I think what most of us are saying is to try to keep it at 190 all the time under all conditions is a tall order.

The fact that you had the same issue with a slant 6 shows maybe it's not an issue

If you look at any stock temp gauge there is a range for normal. The low end would be thermostat temp and the high end would be the upper limits of the engineers design for tolerances etc. With a safety factor tossed in.

To test your air flow out therory, take the hood off.