Hi Oscar ! Welcome to The Kintyre Forum.
Going that far back you're obviously a bit of a specialist ! Are you on Ancestry ? If not, I only saw 2 Janet McMurchy's married to Peter Stewart and the info is sparse with no death dates. (so they are certainly not you) Anyway, one of them provides 9 children from William to Anne.
Forgive my ignorance but what's a box grave ? If it has a headstone perhaps you could research the Ralston site. If you find a name and date that corresponds, it’ll give you a reference number..., as such, just contact Muriel in Edinburgh and she’ll send you a free image with, I hope, some family info.
http://www.ralstongenealogy.com/personquery4.phpOf course, the advantage of this site is the fact that you just might find some Campbeltonian Stewarts or McMurchy’s connected to your research.
Good luck..., Iain.