I'm wondering if the Indian Child Welfare Act of 1978 might come into play and assist with this. Up until that point, up to 25-35% children were placed outside of the tribe and never reunited with family. The goal of the Act was to keep children in their extended families and communities. This child could have some records from this time if registered on a reservation. Either as one whose family are looking for her or one who was indeed placed in an extended or tribal foster/adopted home or one whose placement violated this. This time period was very volatile due to historic abuses and many records were kept leading to the point of assuring reunification or keeping children with in their tribes.
Another thing to note with the isotopes studies is many first nations tribes/territories and do extend into Canada. The external environment is very similar quiet north and definitely does not end or abruptly change at the US/Canadian border...extending our search upwards might be a good idea. Thoughts?
http://www.npr.org/2011/10/25/141672992/native-foster-care-lost-children-shattered-families