Could be that your water channels in the engine block are full of crud,,,,,if it's not that,,then i'd be looking at the volumetric efficiency of the radiator you have in your vehicle.
An electric fan is not going to help you at 80mph,,,,,you should have sufficient cooling to do the job at that speed even without a fan.
I'd definately look at increasing your rad core number,,,,and give your engine block a realy good flush.