Typically, the fan is not involved at highway speeds unless the configuration has a lot of restriction to air flow. Highway speeds are generally able to move more air through the radiator and engine compartment due to movement than the fan is able to impart.
A common issue is "chalked up" radiator (internal) tubes which are too restrictive to flow volume and in which the deposits ("chalk")insulates the coolant from the tube and so reduces heat transfer. The radiator may appear to be clear but having it checked by or pull and take to a radiator shop. They should be able to flow test the core for volume.
+1 on ronfisherman's reference to the lack of a thermostat. No way to know what you mean by "I have been through the cooling system" but please do not take offense by the suggestion that you may not have the expertise to identify issues. We used to see this frequently when was still in the business as the causes are not always evident to the inexperienced. The problem created by not having a thermostat installed is not excessive coolant flow rate but rather turbulance and changed flow path.
A bad thermostat is a common cause of the symptoms referenced because a thermostat which has a partially depleted wax pellet will be unable to open completely. Any time and engine is overheated, IME, the thermostat should be replaced.
Another possible is that the lower radiator hose is collapsing and so restricting flow. These hoses usually have a coil spring inside the hose or a reinforced wall structure to resist collapsing under "suction".
The Chrysler V8's of that era were bad for filling the cooling jackets with sludge/mud and often would benefit from flush using a back-flush machine using air bubbles.
If you have not compared radiator inlet & out;et tank temperatures under load, you may be advised to do so as this will indicate heat transfer efficiency.
Another possible is that there is so much heat introduced into the cooling system from the automatic transmission that this is driving the cooling system over its ability to extract heat. A large transmission cooler can be the solution to this issue, but again, measurements by an experienced professional may save you money in the long run.
Of course there is the possibility that the engine is not actually over heating....? What are the indications of excessive temperature? If the dash gauge, have you checked to ensure that the temperatures are actually high?
Hope something here is helpful.
