Too lean?

Overheating on the freeway is a flow or volume issue, no matter what you have already done to it.
Just because it was flushed only means there isn't anything loose in there, and does not mean it isn't restricted still.
A shroud will only help in town and does nothing on the hiway.

You may have to bite the bullet and replace that radiator (after everything else is eliminated of course)
I totally agree with the part about the OEM temp thermostat.

Someone ^^^:D didn't read your post before they replied.