HOLLYWOOD (AP) — Marriage in the best Hollywood tradition merging wealth and beauty—begins unpretentiously Thursday for dime store heir Lance Reventlow and actress Jill St. John, Reventlovv, 24, a playboy with a flair for auto racing, reputedly is heir to more than 20 million dollars. This will be his first marriage. Miss St. John, 19, a red-haired starlet who has been in the entertainment business since the age of 6, was married at 16 to a linen supply company executive and divorced at 18. Her press agents say she has an IQ of 162 (genius rating). The young couple say they willbe wed in a civil ceremony but won't say where. They got a mar- rage license at Riverside Tuesday, The small group attending the wedding will include Reventlow's mother, heiress Barbara Hutton, and Jill's parents. Mr. and Mrs' Edward Oppenheim of Beverly Hills. Jill said her wedding ring will be a plain platinum band. "Anything more elaborate wouldn't look good because of the size of my diamond engagement ring," she said. "I don't know how many carats it is—I never asked Lance, but it looks big. It goes almost from knuckle to knuckle." It will be a single ring ceremony. Lance can't wear a-ring because he works around his racing cars so much," Jill explained. "I think people will know anyway that he is married. She said her husband-to-be is so enthusiastic about car racing that she had to make him change out of his racing overalls when they got their marriage license. Honeymoon plans are indefinite because Jill has to return to work on a movie if the actors' strike ends. "As a matter of fact." Jill said, "I don't even know where we'll spend our wedding night. I guess I'll leave that up to Lance.