Matt,
If you are after stiffness, whether in bending or in torsion, you have greatly handicapped yourself by using 3/8" core material. That means you only have 3/16" from the neutral axis to the outer fiber in bending, in the terminology of a structural analyst. For a 60-sized plane?
That means that to regain the stiffness that you would have had just from 1/2" balsa you're adding weight. And for a 60-sized plane, most guys complain that the fuse isn't really as stiff as they'd like it.
You're going to need more than polyspan to get that back. You may find that even carbon fiber strip will buckle on you if you get a lateral shock load. Think hanger rash, or....
Cutting away core material in a plywood skinned construction may reduce the weight some, since the plywood is going to carry all of the lateral bending loads due to higher modulus and location, but the issue then becomes how much can be cut away without allowing the plywood skin to buckle and separate. And, you have more than just the lateral bending load case. For torsion you need a closed tube to be efficient. A tube with a horrible section aspect ratio of 4 or 5 inches tall and 3/8" wide....is just not efficient. To carry the side skin stresses from one side to the other, the top and bottom balsa sections will need to be pretty deep.
If you want an example of a profile fuse on a 60-sized plane, go look at Frank William's Bearcat. Plans were in Model Aviation. I don't think the core cutouts were completely optimal, but they worked.
Dave