Four blade, versus seven blade fan in my 225 Duster? Final report now in.

The 4 blade fan is a "fuel economy" special. Low blade pitch, doesn't move a lot of air. A/C and HD models got more blades, but the effect was mostly from the higher blade pitch. You probably can't detect the noise difference on the highway, but they will be noisier at idle. As you would expect, they will move much more air at low engine speed.

I used to have a 7-blade fan from a 383 on my Commando 273 — at fast idle it was like a hurricane... blew dust and papers down the street. I replaced it with a clutch fan to see if I could even slightly improve the fuel economy. Never had any overheating issues, though.

On my last slant I installed a Texas Twister flex fan. That did seem to help with cooling in hot weather. I didn't have a shroud, either.