Here's a little advice on how to find out about your local drive-in theater. Your local library is an excellent resource for finding information on drive-ins in your area.
Most libraries have rolls of microfilm of old newspapers on hand.
Look for the advertisements for the movies that were playing at that time. For the best of luck for finding drive-ins is to start with newspapers from the mid to late 1960's. Of course you want to check the summer papers, not the winter ones. Most drive- ins were closed in the winter and not many are actually open that time of year. The biggest advertisements were usually placed on Fridays and Saturdays.