My 6.0 LS swap runs 210-215 degrees at highway speed despite an upgraded radiator and electric fan. How do I get the cooling temps down?

Running hot at highway speed (rather than in traffic) suggests the fan isn't the main problem — moving air is doing most of the work at speed.

Where Speedway says to look next:

• The tune: a lean air/fuel mixture makes cylinders run hotter, and ignition timing that's too far advanced or too retarded also adds heat. Speedway notes improper timing or air/fuel can make an engine run hot even with a properly sized cooling system.
• Water pump: a worn or low-flow pump may not move enough coolant for the power.
• Air in the system: make sure the steam line is plumbed and the system is purged.
• Airflow path at speed: a shroud can restrict ram air at speed — performance shrouds use bypass flaps that open at speed for this reason. Also check that air is actually getting to the radiator (openings, fins, debris).
• Radiator capacity: more rows help with real power, but add thickness and restriction.