Yes, balsa for a model of this size is fine. The size shown, which I didn't label, is that the spars are done from 1/8" sheet of a medium weight. The leading edge sheeting I mentioned as "soft" can be medium weight and strength as well. "Soft" is intended to refer more to the sheet's ability to easily take the curve.
The fiberglass and epoxy taping around the center section will do just fine for dihedral loads. It's the same method you would have used with the foam wing in any event.
I missed the notation about ailerons. And certainly an SST would have ailerons. Instead of bringing the lower TE sheet back and shaping it to a point as shown in the sketch do something more like shown below with the cap adjusted to the right point for the ailerons. Adjust the rib outline as needed to allow for this clipped construction. I don't know what the width of the ailerons are so this is more of a concept idea which you'll need to adjust to fit the right width ailerons.
Also now that I realize that this is an aileron and elevator controlled model I'd suggest that you don't need dihedral. As such you can build the wing all at once with the 33" span and add some wing tips and call it a day. So just make the spar longer and use a single rib in the middle and sheet across the center section to cover two rib bays.