Thoroughly enjoying this thread but it's taking me a while to read through - very slow reader.
Glad to see Jim Carroll thinks the Big Boys are treated with too much reverence. I do too. They should be respected, but singing some of them verbatim would take a bit of doing. But above all they should be enjoyed. I've been singing them for 40+ years and still do so whenever I get the chance. These big songs are stories - and stories are, I believe, what make us. To my shame I have never done a great deal of research on the ballads but I love them and believe that changing the words slightly or altering the tunes to fit are not horrendous crimes. How else did they evolve, grow and travel to so many countries in so many different versions?
These big songs are not only enjoyable and "meaty" to sing they teach us that there is often a payment to be made for our indiscretions.   
In singing ballads my preference is to fit the tune round the words - ie follow the rhythms of speech rather than tune. This to me gives a more satisfying result for both singer and listener. Like Brian and Jim, when Anne Neilson and I run our ballad workshops we encourage people to speak the words over as though it were a piece of prose and find the stresses that naturally occur in speech. Then start fitting the tune around it. It's what Anne and I have done for more years than either of us care to remember and I feel we both sing these Big Fella's tolerably well.