Hi Richie,
Like your own site the Roud Index is a massive project. Unfortunately it doesn't really cater properly for hybrids. At first Steve started putting related songs all under one number. I helped a lot to change this and sorted out some real messes like all the Died for Love relatives and they now all have separate numbers. I have just requested that Steve give separate numbers to 'Wild and Wicked Youth' and 'The Flash lad' the reason being that though they have 3 and a half stanzas in common out of about 12 stanzas in each song that is much less than half the total. My own personal response to your question is go with the majority, i.e., if the song contains more than half of its stanzas of song A then classify it with that song. Another alternative is to give both (or multiple) Roud numbers where hybrids occur. 327/1168, or even explain that the song is a hybrid.

As I've explained before, where a new song has arisen from the hybridisation of 2 or more songs and can be demonstrated to have become established in oral tradition, or where a new song has arisen from the ashes of an old one, American scholars call this 'oecotype' or sometimes 'oikotype' and treat it as a separate song, e.gs., 'The Butcher Boy', 'Streets of Laredo'.