overheat only over 3000 rpm

Uhh, you have 4 pusher fans in front of the radiator?

I know you've broken this down into RPM, but I assume that this "overheats over 3k RPM" probably translates directly to a highway speed as well. As in, it doesn't overheat at 3,000 rpm in 2nd gear at lower speeds, but only at 3,000 RPM out on the highway at whatever speed that translates to?

Overheating out on the highway is usually an airflow issue. And if you have 4 pusher fans, well, they're blocking your airflow.

What is the combined CFM of all your fans?