my friend has a dc-3 on the building board and i mentioned your question, his reply was you are absolutely right about not cutting down the spar, you don't want to do that. butting the sheeting up against the spar with no gaps whatsoever will work just fine. make sure you have plenty of glue on the edge of the sheeting contacting the spar. this arrangement should provide plenty of strength if you are concerned about further structural integrity you could run a 1 inch strip of carbon fiber tape with epoxy thinned with denatured alcohol about the consistency of milk over the full length of the spar.