I respectfully disagree. When these trucks and cars were new, traffic was not what it is today. They were never designed to sit in the kind of traffic we have today or a parade on a really hot day. So when first designed, the four blade fan was more then adequate and frankly less expensive to install on the production line. .50 cents saved for each car or truck built over the course of say 1.2 million production is a $600,000 dollar savings. So GM was certainly trying to use the minimum necessary.
I recently installed a five blade fan from a 59 348 motor with AC on my 261 and the difference in air flow through the radiator is significant. I sat at idle in a line of vehicles leaving a show in Bowling Green KY a couple of weeks ago in 98 degree weather for a significant period and the motor never got beyond 185, a first in many many years. With the four blade fan, I would be just under 220 pretty quickly. Just going from the four blade to the five blade AC fan alone reduced the temp to a very manageable level. So my first hand experience tells me the additional blade is very helpful to keeping these motors cool for today's usage.
