After everything is finished, I think there is an unintended error in the grid. I have checked several references to the poem on the internet and they all agree on the poet's spelling of a 7-letter word appearing in the top row. I would be surprised if that is intended. Or perhaps there are other versions of the poem with that word spelled the way it is in the grid. It does not change the validity of the instructions; it just reduces the number of words of the theme by one. Since we are not asked to highlight those words, this will not affect solutions.
I am a bit surprised that two other words of the theme did not appear, since you get real words by applying the same exchange used everywhere else.