What is the best way to get an overdrive auto in a 67 Barracuda?

Like I said, it is over 3k rpm cruising down the road at 55-60 which is not comfortable, we go from what is the best OD trans to fit in an A body to what are the precise numbers of your rpms vs mph? Like I said who really cares? Anyone who has driven one of these cars with 26" rear tires 355 gear and a 727 trans knows good and well what I am talking about. It doesn't take a smart-*** math-magician to figure it out.