if you sheet with balsa, you won't need FG. it's just needless added weight IMHO. if you want FG, I would cut a core, sand to shape, spackle, then FG. the problem with a cut core, is that you've got to hollow out sections for servos, flap recesses, aileron recesses, etc.. if it were me, I would do a built up wing using foam ribs, with balsa, hard wood and CF for stringers and spars. with a wingspan this large, I would be very suspect of the structural integrity of a foam core. if you look at super large scale builds, more often than not, they use a built up wing because A) it's lighter than a foam core, and B) it's stronger than a foam core of the same size. the bonus is that you can build everything right into the wing and not have to reverse engineer a cut foam core to make everything fit right.
this is your decision to make here, but thought I'de point out some pros and cons of foam verses built up. the obvious pro to a cut foam core, is the ease of manufacture. it's the easy way out, but not the best way out. there's lots of ways to do a built up wing, if you think it through and design it on paper, or computer.