Have you checked the coolant temp with a thermometer? The fan shroud can be a big help, too. Also, do you have a pan in front of the radiator? Without a pan, air can go through your grill and down to the road. It can go up above the radiator if there isn't something forcing the air through. What do you have for a fan? I know some of the earlier pickups had a small fan, but am not sure about the bigger trucks of this vintage. Used to be you could go to the parts store and get a 5 blade fan for these, which was a big help.
