Cindy Williams was born in 1947, and was 26 when American Graffiti came out. Ron Howard was born in 1954, and was 19, about the actual age of a high school graduate when the movie came out. Williams said it was tough playing a high school junior, about ten years younger than her actual age, and pretending she was younger than the nineteen year old Howard.
You are correct that Harrison Ford had no significant scenes with Cindy Williams in the film. But the girl and the guy who met that night and cruised around were Mackenzie Phillips and the junkyard guy.
Regards,