Many things factor into cooling system efficiency, and not everything that solves one persons problem will necessarily solve someone else's problem. All the pieces have to be put together to form a system.
The first gen birds do have a limited frontal area same as the 67 GTOs did. In 68 GM widened the frame horn area to allow a larger frontal area for the radiator on A bodies, but F bodies didn't get a redesigned subframe until 70.
The narrow crossflow radiator did one other thing beside have vastly smaller frontal area, the small radiator also reduced the cooling system capacity. One of the things I learned when trying to make my oval track cars run cooler is additional capacity also helps. Ford used to use the reservoir expansion tanks in their systems as it added more capacity to the system.
GM/Detroit diesel 6.5 engines had problems with O/H usually pulling a load on long grades. Lots of warranty dollars got spent by GM because of the marginal cooling systems in those trucks.
The engineers made several modifications to the later truck engines to address the marginal cooling systems. Redesign the clutch fan to come on at a lower temp, and lock up more positively. A water pump impeller redesign to deliver approximately 40% more coolant, dubbed the HO pump. In conjunction with the water pump improvement they made a dual thermostat water crossover to allow the redesigned pump to not be restricted at the thermostat housing. These changes made a lot of difference in the cooling efficiency. Many of those changes on the 6.5 carried over to the 6.6 Dura-Max engines.
Picture of dual thermostat 6.5 water crossover:
Another thing is the whole cooling system in these 50 plus YO engines and cars have crud and sediment in them, that may or may not get addressed during restorations. Some of these cars are still using the OEM engines the car was built with. Lots of variables in every car/engine combination.
Chrysler had their K cars with 1.7 VW engines in them with a iron block, an aluminum cylinder head, and a brass radiator. Back in the 70s the cars would O/H on the highway due to the corrosion combination of these 3 metals in the cooling systems the radiators would scale on the inside effectively insulating the heat rejection of the radiator. It took place in under 5 years and around 40,000 miles.
Lots of old wives tales on the physics of how an engine cooling system works add to confusion when troubleshooting cooling system problems.
Lots of things that most people haven't a clue about that take place under the right conditions in older cars, any combination of some of these things can make you tear your hair out trying to solve an O/H problem.
