70?? damn, yo... all my 6 string friends think my .56 is pretty bulky. How's the playability when you get that thick?
If you're tuning fown to B, i'd actually recommend doing it this way:
That way, your 6-string chord voicings and scale parrens will be the same- it'll feel more natural to you. And i mean, it's a simple tuning change to go between this and a "bottom 6 from a 7" tuning- only one half step difference on the F#/G string...
You could definately set up a floyd-equipped guitar this way, too. you'd need to re-intonate, of course, but that and maybe another trem spring and you're in buisness.