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you want to block it as much as possible while maintaining cool air for the intake and an opening for the transmission cooler. beyond that as long as your cooling fans stay off the majority of the time and have enough of an opening to pull plenty of air when they come on, you'll be ok.
it is worth mentioning that you can add a water injection system that, on diesels, will help you keep cooler ECT temps as well as add a bit of power.
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