In your favour, goffik, Camden's Britannia has probably the first written record of the name and says 'This stone the common people there by dwelling, name Long Megge, like as the rest, her daughters'. So the word 'long' is specifically in the name and that was 1610. But I'm not sure the person he got the story from used the word 'long'? Actually the more you look at this the more complicated it gets.

I myself don't hold with the meldon theory, these books have got lots of specifically meldon-based stories about meg of meldon, which makes me think she's not a vague enough figure to be attached to a circle in cumbria (and anyway meldon is not local, it's the other side of the country, just north of newcastle). Besides, if she really was Margaret Selby, she'd have been around in the 17th century, later than Camden's mention of 1610. There's not much in the stories to suggest any witchiness really either.
http://books.google.co.uk/books?id=yhcHAAAAQAAJ&pg=PA135
http://books.google.co.uk/books?id=_rtV_hzhDx0C&pg=PA711

So what connection does meg of meldon really have at all? I don't really see one. At least Long Meg of Westminster is Long. She's definitely earlier from the 16th century and the reign of Henry Viii, featured in popular culture in stories and songs and so on, and there was a popular saying 'as long as megg of westminster' for people that were very tall and thin. Westwood and Simpson in 'lore of the land' are going for the long meg of westminster theory.

and there's lots of interesting excerpts that relate to the westminster version in this book
http://books.google.co.uk/books?id=IoW9yhkrFJoC&pg=PA80
although it doesn't sound very promising by its title, actually it's not yoghurt-weavy as you'd think. They don't really look at the meldon thing at all though, dismissing it as too late to be relevant. They conclude (I think) that there was already some story about the stone being a woman, they don't think the meg of westminster thing would have got tacked onto it so easily otherwise. But that's when you start wondering about the relevance of 21st century views of paganism and goddesses and prehistory and so on to the writer's argument. Anyway read it and you can be confused too :)