Don't forget the dynamics of the radiator. I would think there would be a temp difference top to bottom. If you are thinking the tubes are plugged, short of pulling it and having it boiled, cooling system flushes would be the choice.
You can look into the filler neck and see the the top header and tubes..it's an old truck, they will probably be partially plugged. Pulling the radiator isn't a huge task. Make sure it is the culprit.
Stuart
