Overdrive auto for a 383 72 Dart

Just joined here and I'm trying to find out best fit for a 72-383 Dart currently running a 727 in front of 3:55 's just not happy with the RPM at cruising speed, I've researched Kiesler and discovered they declared bankruptcy and I don't want a gear vendors bolt on overdrive unit been told to do up a A518 been told 700R also 4l60E any help from anyone that has traveled this road would be greatly appreciated the car is running 500 hp