You mean "yardage/pound" not "weight/pound" because they all have 16 ounces to a pound.

9 st/in on #3? That's insane. That's not a sweater, that's a suit of armour! Is there a Ravelry group for it?

I suggest you get 1 ball of the yarn you're thinking of using and another in a lighter weight and swatch. :D