Should I explain the origin (and permutations) of this stanza, as it appears in Uncle Tom's Cabin, with a note, or with an article?

It is a beautiful belief,
That ever round our head
Are hovering, on angel wings,
The spirits of the dead.

Annotators of Uncle Tom's Cabin agree that we don't know where she got it from, or whether she wrote it herself.

I in fact know. It's not from William Cullen Bryant, though it was attributed to him in the Era. Nor is it from Christopher Pearce Cranch, to whom it was also attributed.

It's by Charles Hansayd Perkins, except that apparently he revised it, because William H. Channing omitted the relevant stanza when printing Perkins's Memoir.

Also, the reason it was attributed to Cranch is because he wrote an echoing poem, which also appeared in the Western Literary Messenger.

It's attributed to Cullen Bryant, because newspaper editors do what newspaper editors do, as @ryancordell and Abby Mullen explain.

And it appears in the Poems of Cullen Bryant, on Google Books, because Ralph W. Kirkham (or his wife) pasted the copy of the falsely-attributed poem in their copy of Bryant's Poems, after cutting it out of the Minnesota Pioneer.

The readers of the Era likely thought it was by Bryant, because it was reprinted in the Era, attributed to Bryant, while UTC was running in serial.

Oh, and of course Stowe altered the words, in her various uses of the stanza, including in "On the Ministration of Departed Spirits." In no one of those did she name stanza's author.