The Bible is a unified book with one Author, God, who uses words interconnected in specific literary ways to communicate His message. The Flood perfectly exemplifies how truth can be gleaned from careful study of literary clues in the text.
Genesis 6-9 are unusual in their level of careful detail, compared to all the preceding chapters. In Genesis 1-5, God accounts for about 1,600 years of history with very little narrative detail. Yet God takes four chapters--a snail's pace--to set the stage for and describe the single year of the Flood. Several clues point us to the reason why:
(1) The exclusivity of God's salvation: Only Noah and his family were saved out of an entire world of people that were in rebellion and under judgment.
(2) The grace in God's salvation: Surprisingly, God spared the animal "kinds" that lived on land. God could have destroyed the animal world and then re-created it, yet He chose to save the animals on the Ark.
(3) The success of God's salvation: Judgment and salvation were accomplished exactly as God planned. The Ark didn't sink, the animals didn't starve, and the passengers and crew didn't suffocate because God knew and planned for every detail of the Flood--including the exact size of the Ark to hold all the animals and people for a year (today's verses).
Peter echoed this message (2 Peter 2:1-9) to encourage his readers to trust God. If God didn't miss a detail in Noah's salvation, surely He can save you. God doubly encourages us to trust Him, both by Peter's exhortation and by the level of careful detail in Genesis 6-8. NTJ