I am sorry to say but if you have over heated it (especially more then once) you have probably warped the head and the head gasket is probably blown allowing coolant to pass over into the firing chamber, that seems to be a problem with the straight 6 because they are such a long motor, have you noticed a lack of power, white colored exhaust, milkiness in the oil?
This is probably why it is still overheating on you at highway speeds the motor isnt running right making it work to hard and water in the oil makes for less lubrication making excess heat