In the book "Jack In The Box", Jack Twiller's experiences are followed from elementary to high school, while in his screenplay, William Kotzwinkle creates a younger brother, dividing these experiences between two separate characters.
When the Chevy convertible is being pulled out of the water, you can see by the grille it is clearly a 1953. For the rest of the movie, it's a 1954 Chevy convertible.
