The OP, I think, was looking specifically for material for use in the “New World,” or the actual landmass that would become the Americas, but I think this remains a useful observation for this topic.
The article I link is in this line of inquiry. I am ashamed that I haven’t read it yet, but it could be of interest here.
For Americana Conan, I also think of James Fenimore Cooper’s Leatherstocking Tales. I read them all a few years ago, I really enjoyed them, and I can see their influence on REH’s work, particularly on “Beyond the Black River.”