Cross-Country Road Trip in a '68 Barracuda

General rule is if it gets hot in town or idling its the fan, if it gets hot on the highway its the radiator. Go to a car wash and power wash the radiator real good, its amazing what gets stuck between the cores. Pulling the thermostat may help for a short distance but, not a long haul like your doing. Without the thermostat the coolant never stops moving so it never sits in the radiator to cool down. The longer it runs with no thermostat the hotter the water gets and, it eventually over heats. I would wash out radiator and get the coolest thermostat you can, maybe drill a couple 1/16 holes in the outer ring of it and hope for the best.That flexolite fan aint helping either.