Of course, I know you were kidding in alluding to The King and I but Gertrude Lawrence could never have been a consideration: she died in 1952. But maybe Robert Harris was referring to Ethel Merman in Annie Get Your Gun or Panama Hattie or Gypsy.
Well, to be fair, though Tandy was the original Blanche in New York, Leigh was the original London Blanche, so it's not like she wasn't familiar with the role.