That's not such a good plan, because the AMCAS primary application for Fall 2019 starts in early June, 2018. You ideally want to apply sometime in June to maximize your chances. Taking an August MCAT means that your score won't be available until September (about 30 days after your exam date). Many schools will not review your application until it is complete (has an MCAT score), so it will sit dormant while other applicants are being reviewed, sent secondaries, and offered interviews.
Is there any way you could take the MCAT this May?