If instead of Sally Ann Howes, you had to cast an American actress to play the female lead...

to "better" match with Dick Van Dyke (who was using his natural voice unlike in Mary Poppins) who could it have been? I would immediately think about what Shirley Jones would've been like as Truly Scrumptious. She was of those singers/performers you could say that fit in "Julie Andrews" (speaking of Mary Poppins) mold who are simply likable, gracious, sweet and have a wonderful energy about them.

https://i.pinimg.com/originals/ba/0e/79/ba0e790ed29b1bfed9ab9fce8e6592a6.jpg

https://www.youtube.com/watch?v=Vnxgylafhbk