5 or 6 speed auto on a 360??

so when i get to pick up a tranny for my 70 dart, i was thinkin maybe getting a 5 speed auto. i understand that with more gears, i can shift at lower RPM giving me better gas mileage. do they make maybe a 4,5 or 6 speed tranny that can hook to an old carbed 360? im lookin at only an auto, since this is my daily diver and cruiser. ill do a 4spd manual on my next project i promise!~8)