If I replace my 4 blade radiator fan with a 5, 6, or 7 blade fan will I get more engine cooling?

I have a slant 6 motor.

A fan shroud will help the most. I don't have a shroud. I installed a 7 blade or maybe it was a 6. My 64 D100 is definitely cooler. I intentionally pushed it to 65-70mph and stayed cooler than it ever has.

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I'm going to attempt to fix this project and not make it any worse.

Short answer is yes, it will have a positive effect, at low speed.
Whether it'll be enough for your purpose is the subsequent question.

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Question is too vague to get a helpful answer. Article to read is here (eventually you'll get to Part 5, which covers fans)

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