If you have incurred business expenses in 2018, I would suggest that you file Schedule C for 2018, even though the income for your project was received and reported as income in 2017.
It is entirely possible that you have deductible expenses that you have not considered. Do you have any business assets acquired in 2017 or earlier that should be depreciated in 2018? Are you using an office in your home for your writing, you would want to take that valuable deduction in 2018.