Mick,
The Fletcher poem that Joe posted is from "The Complete Works Of Cole Porter," not from the sources you cited.   
From the narrative given in the "Complete Works" book, it sounds like Fletcher probably wrote "Open Range" first, and then when Lou Brock approached him about writing a song with the title "Don't Fence Me In" for the planned movie "Adios, Argentina," he expanded on that poem's theme to write a complete song -- lyrics and music - called "Don't Fence Me In."

From Kimball's book: [[ On November 24, Fletcher sent Porter the lyrics and music for the refrain of "Don't Fence Me In" and some lyrics for a verse. On December 1, Brock wired Fletcher that "Cole Porter wants to buy right to use title and some characteristic words and phrases from your lyrics. ...

"Fletcher complied with Brock's request, and Porter accepted the offer ...

The agreement ... gave Porter all of Fletcher's rights to the title and lyrics of "Don't Fence Me In" ... . The agreement did not apply to Fletcher's music, which Porter did not use ... ."]]

It's pretty clear from this that it was not the poem "Open Range" which Porter bought from Fletcher. That poem does not have the title that Brock wanted and that Porter bought and used. It also has hardly any lyrics or lyric phrases that Porter used, and it did was a poem - no music - not a song.