alternatehistory.com

I need some help with something.
I'm currently writing a story in ASB where Superman ends up growing up as a Sioux, and he will eventually lead a spiritual/racial war against the US during the Civil War.
Unfortunately I have not been able to find much useful information about the number of American Indians living in the United States in the early 1860's. So does anyone have any reliable numbers for the Indian population at that time?
Any help would be greatly appreciated.
Thanks.
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