408 Magnum stroker running hot.

At 25-40mph, you are getting the absolute best air flow and a minimum amount of heat generated by the engine since it is low rpms.

The water pump pulley should be at least just slightly smaller than the crank pulley to create enough pressure in the system.

With overheating at speed, you can narrow it down to head gasket leak, malfunctioning wp, timing issue and not enough volume in the system.

Assuming you have dealt with the other issues, It is possible that the Champion radiator was clogged from the factory. That isn't impossible to happen. It is also possible that a 3 row is not enough volume for your motor.