First, I hope you understand you are going to a drive-in and not a theater. Second, you did not mention what time you would be going to see a movie, the movie you are going to see and the day you will be seeing this movie. This would be good information to provide to help people answer your question more specifically. Third, the definition of a "good spot" is subjective. What might be considered a "good spot" for you might be different to someone else. The spot you like might be available without having to be there early. If you plan on bringing food in they're pretty chill but as a courtesy you should purchase at least something from their concessions so you can support their business to keep this great thing going. It's pretty reasonable and it's not bad for "movie food".
You will most likely move much closer after the racing fans leave. It empties out after the races are over. And you can bring anything you can fit in your car. They're not picky here.
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