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My guess is that you '84 has been running fine without the deflector but not necessarily at optimum cooling. I'm no expert but I think the purpose of the deflector is to spread the air around to the various cylinders. Not having it probably won't cause immediate damage but might make the difference of an engine running 350,000 miles vs 300,000.
Myself being anal, I would replace it. I hate having missing parts.
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