There are two different car models being shown as the car driven by the sisters, as you can tell by the tail lights.
At the beginning, the episode flashes text on the screen that says "Tucson, Arizona 1964." This was incorrect. It was actually 1965 and even the show's own press release contained a synopsis that quoted 1965.
Charlie Schmid's car is a 1968 Chevy Chevelle and this happened in 1965.
The film the girls went to see was not the one in this episode. The real movie the girls went to see was Tickle Me (1965), starring Elvis Presley.