the point of the recommendation is that it's a fact that if the fan is too far inside the shroud or too far outside the shroud, the efficiency plummets.
^ This. It's easy to prove on electrically driven fans in a shroud, like your outdoor fan in your AC condensing unit for your house. When the fan is halfway in the shroud, minimizing the gap, the amp draw will be higher = fan is doing more work. Look at the fan position on original or restored cars.
Low speed cooling issues are usually airflow, with a semi-fouled radiator second. A rad that is "plated out" with scale or rust doesn't flow well at low water pump speeds. As the rpm reach cruise, the differential pressure increases and the flow improves. The real high flow 'stats made a world of difference in my 383. Look for an opening of about 1 7/16" diameter.
