She played nuns a couple of times. How about Anne Shirley?

That’s true. The other obvious go to actress for a nun character would be Deborah Kerr, but I’d rather use her for a more important character (maybe a Tully or Septa Lemore, who btw might work as Loretta as well). I must say I’m not familiar with Anne Shirley’s filmography at all, but she might work. I’m considering Anita Louise as well.

Loretta Young? Lord knows that was her schtick….or do you want someone blonder and younger?

OMG, that would be hilarious but I’m afraid it would only work if you know Loretta’s background. Plus I was considering casting a young Loretta (around 1928) as another character, given Loretta’s story of giving-birth-to-a-child-conceived-in-problematic-circumstances-and-whose-parentage-was-kept-hidden-until-said-child-was-an-adult. But tragically-died-before-her-time-in-mysterious-circumstances Natalie Wood is also an option for that character.