With Marc's advice, my buddy and I strengthened the base of the fin where the bolts go through by inserting 3/8" dowels through the balsa, and a wedge shaped piece of spruce inside the fuse so the blind nuts will sit at right angles to the bolt.
Because we are using a heavier engine, and seem to be adding weight behind the cg, we are going to epoxy the fins on, using the bolts as clamps. When dry, we will pop the bolts and blind nuts out, and epoxy carbon fiber shaft to fill the holes. Hopefully, at that point we can remove some of the added spruce wedge as well

The wings and fuselage will be covered in Ultracote, and we may possibly do the stabs and fin with glass - but that hasn't been finalized yet...