I was completely unfamiliar with the poem featured this week, so very grateful for the Internet. It’s a particularly long poem, and my way into finding the quotation was to identify the letters that needed to change for the first three across clues, which made a three-letter word, and then find all occurrences of the word in the poem. I found one plausible phrase that, if you omitted the last word, consisted of 44 letters, which is the number of clues. This helped to complete the last few words.
By the way, the correct letters in the across clues spell out the title of the poem and the first name, middle initial and last name of the poet.