@Randy I don’t think there can be any better advice than to just pick a D and see what it feels like. If you don’t like it try another.
We are all different, so the answer that is best for you is for you to find. This forum is called “Improvise for Real” after all, so I assume that improvisational exploration is the approach that people drawn here will be most likely to follow?
I hope that helps. 
I don’t know much about Chord Melody, but I do sometimes try to play songs on my guitar with a bit of accompanient, so here’s how I would approach it.
Try to work in tonal numbers, the melody and the chords. Play the melody somewhere on your guitar, note 1 being D is as good as any. Play the melody and try to find some notes from the accompanying chord to play with it. So, for D chord find notes 1,3,5 under the melody note and try to play these wth it.
Your comfort is key. David’s answer is on point and I can’t really give you further advice than that. But I just thought I would point out that your notes are a bit off. In the key of D, the first note of the melody is A, which is the 5 note in this case. So the first line for example is actually:
How Many Roads Must a Man Walk Down
5 5 6 6 6 5 3 2 1
Sing it and notice how the melody resolves on the word “down”. This will eliminate that b7 you have in there, as all the notes in this melody are diatonic.
Thanks, I will try that. Have you taken the chord melody course 1 and 2? I am looking for someone to work with to help break down songs and play the chord and melody at the same time. Let me know if you are interested.
For what it’s worth, if it were me I’d try it in the key of A and take advantage of the first melody note being an open E on the first string. Plus you have the advantage of having an open string root for the I, IV, and V, A, D, and E chords. This may make it more “guitaristic”, especially if your goal is to play it in a Travis-picking style. If you still want the sound of the key of D, you could capo it at the 5th fret.
That said, if I wanted to keep it in D, l’d most likely pick a 5th position 4-string D chord (with an open string D as the root and play the first A melody note on the first string. Hope this helps!