Britney Spears very nearly snagged the lead role in “The Notebook.” Casting director Matthew Barry told The Post that the 2002 nationwide search for the role of Allie — in which he saw Jessica Biel, Claire Danes, Scarlett Johansson and more — ultimately came down to Rachel McAdams and Spears. “It was tough because we did narrow it down to Rachel and Britney,” Barry said of the effort to find the perfect leading lady for the romance movie based on Nicholas Sparks' novel.