You'll go to the cattle call and fill out some forms. They'll take your picture. I wouldn't wear a costume, but it couldn't hurt to do a hint of the era. You don't want to look different from your headshot.
Expect to spend most of your time just waiting around. You'll fill out paperwork with your name, measurements, availability and contact information. You don't need to spend a lot on a head shot, a simple picture of yourself is fine. They'll take a picture of you if you don't come with one. If you want to dress in a way that suggests the 70's or 80's that would be fine, but I don't think you need to show up in costume or anything. You won't find out then. Extras are often called at the last minute - like you might be called the night before to be on set the next morning.