Elvis Presley's appearance singing "Houndog" on the Milton Berle Show took place on June 5, 1956. Several of the vehicles used in the movie were newer than this. The pink Desoto (or Dodge) was probably a 1956, but it's not likely that a rural Alabama woman would be driving a new car. The tow truck used to tow the Desoto was a 1957 at the oldest, and it was quite beat up. The red Pontiac at the Elvis concert was a 1959.
In the scene in which Dakota Fanning (Lewellen) is lying in the bed, the character played by Robin Wright Penn asks her to sing her an Elvis song. She sings part of "Can't Help Falling In Love" which was not recorded until 1961 for the movie "Blue Hawaii".