With that information, I would do it a little differently. I would take 3 layers of 1/4" Divinycell and 4 layers of carbon uni and laminate a composite spar. You will of course need to modify the ribs for the 3/4" spar thickness. It will come out lighter and far stronger that a balsa spar, and you can make it straight. There are few things more frustrating than trying to make a straight wing with a warped spar.
Scott