If it overheats at highway speeds it is doubtful to be an airflow problem IMO. You really don't need a fan at highway speeds. On my 88, with a bad fan clutch, I couldn't idle for more than a few minutes without the temperature going up. As soon as I hit 30 MPH and higher no problem.
What can screw you up at highway speeds is the lower radiator hose collapsing. I good system and the water pump actually sucks more than it pushes. You need a spring inside of the lower hose or one of those universal hoses with the ribs built in.
I was having issues with highway overheating and by luck noticed my bottom radiator hose partially collapsing when I revved the motor up in the driveway. Almost impossible to see with the airbox in the way. The spring on mine had migrated down the hose and left enough of the hose unsupported that a section it would partially collapse at high RPM's.
Something else that can screw you up is, a tiny head gasket leak. The exhaust gases collect in the high spots in the cooling system and the bubble messes with the coolant flow. The head gasket leak may be tiny, but it adds up.
Lastly the system, in most years I'm familiar with, needs the heater hose bypass, either the bypass through the heater valve or through the heater core. The coolant that bypasses the thermostat, keeps it working and responsive. Without the bypass the thermostat can open, until it gets cooler fluid, then close until the coolant is heated from the rear and this can take awhile. What you end up with is wild temperature swings, instead of a fairly steady temperature. Having the wrong thermostat housing gasket can cause the same gremlins. I always use OEM thermostat gasket, even if I use an aftermarket thermostat. Or make my own to match the OEM gasket.
One thing you might want to check out is to take the top radiator brace off, unscrew the top bolts on the condenser (being very careful,using penetrating oil) and separate the two pieces. I have found Cotton wood fluff in there, some makes it's way through the condenser, but not the radiator. You can also take off a fan and the front grill and shine a strong flashlight on the back (engine side) of the radiator and see the light through both the radiator and the condenser if they are clean.
If you flush the radiator and condenser with water, don't use so much pressure, you may flatten the fins. If you do, you can buy a fin comb at most any air conditioning place.