A poster for Der Marshal (1969) can be seen on the wall outside Jack Warner's office. "True Grit" was released 14 years after James Dean's death and by Paramount, not Warner Brothers.
In the opening scene, Dean is seen wearing a 1990s style racing helmet.
When Pier and her husband are sitting in the car, there is a Nissan or Toyata in the background.
James Dean spends a lot of time on a 1972 Triumph motorcycle.
During the scene in the garage toward the end of the film, there is a poster from the movie Grease on the wall. Grease wasn't released until 1978.
"Elia Kazan" tells "Jack Warner" that Dean has just won a Tony Award. Dean had won a Theatre World Award, not a Tony.