There's a reason we were using a far-in-the-past PoD - the idea being to have there be a genuinely centralized state of Amerindians, roughly speaking around the Mississippi valley. I don't know if it would be able to get that far west - the Rockies might be a natural boundary for it.
The point is, having a centralized state with contemporary modern technology (and, if possible, agriculture on a large and productive scale) would mean an Indian nation with the power to protect itself.
That's also why I cobbled together the government system I did. It's the far end of a couple of centuries of consolidation and compromise, basically... of the same type which turned tribal groups into nations in OTL, though accelerated by time pressure.
The problem with your model though is it completely ignores the history of the region your choosing. There wasn't even necessarily the same tribes in what would be the southeast States of OTL; which even then had their own form of government that is more complex than just the simplistic "chief". It also ignores that there were kingdoms with large cities along the Mississippi too.
So if you want a Native state that has a chance to hold of the Europeans and control a large amount of territory along the Mississippi basin, here's a civilization you can base it off.