Only overheating on the freeway.

Yeah, let's not just shout out names of parts, folks. That's not gonna help him.

The OP almost certainly has a used-up radiator, a faulty thermostat, and/or other clogs and faults in the wet part of the cooling system. The symptoms do not point to a faulty sending unit, a missing fan shroud, or inadequate system pressure (which can cause boilover but no, is not going to cause a hot-run situation). Ignition timing? Sure, that's on the list of possible causes, but the thing to look for is retarded timing, not overly-advanced.

A fan shroud is not going to help this problem. Fan shrouds are for getting more air across the radiator and A/C condenser when the vehicle is standing still or moving slowly. At road speeds above about 25-30 mph, the forward motion of the car pushes more than enough air across the rad and condenser and a fan shroud is irrelevant.

Low speed/stopped/stuck in traffic overheating = inadequate airflow. High speed/highway overheating = inadequate water flow.