I recently purchased a Dell'Arte Leadbelly 12 string (the hand-built version not the Mexican-made version). I spoke with Alain Cola (info@gypsyjazz.net) and the builder John Kinnard who did an enormous amount of research into this instrument before building it. The strings they choose and used to sell for this guitar are:

14/14 18/18 25/25 16/34 19/46 14/64

I love this guitar by the way.