It is getting the most airflow through the core, and the shroud is a must to do that. It does not have to be large, only covering the height and width of the core, and "edge" sealing the fan to shroud" so the ends of the blade are not stalling/stagnating" airflow.
Another thing about most of our cars (60s-70s) is the buildup of airflow in front of the car and the poor "through flow" of the grill that they encounter. Some going faster on highway run hotter because insufficient flow from the bad aerodynamics of flow in the front of the cars. The air is diverted from the pressure build up and the invisible "bump" of air pressure prevents proper airflow.
Shrouds help ensure as much air as can be pulled through by the fan occurs.
I bought a 1968 Skylark. 350/ST300 and 2.56.
Had a 16" flex fan and was running hot, about 210-220.
Pulled the POS flex fan and tossed it in the trash, and grabbed the 18 "tractor" blade for my nailhead, since it was in the '64 was still going back together.
Temps on the 350 dropped to 190-195.
That difference is the shroud/fan edge distance and the proper "pull" of airflow through the radiator.
I recall back in the old days of cars always overheating because people had the incorrect fan blade, removed shrouds, removed thermostats and that being the largest cause of overheating.
Even though the fan on the nailhead is near 1-2 inches to the core, I wanted the shroud since I live in Florida and mid 90s most of the summer, I figured every little bit of an edge I can give the engine is worth the cost.