Roger, you're referencing the "rear" fuselage, right? I was trying to apply what you said to the front--the engine pod, as someone (Frank?) called it...I understand now. I was referring to the method suggested in the construction article of the original, in the magazine, and as shown on the profile view on the plans: The fuselage cutout for the wing, from the high point back, is cut on a straight line--then before the wing is sheeted, the fuselage is slid up between the R1 ribs and positioned exactly as you want it; then trace the rib curve onto the fuse side that is overlapping it and trim to that line. Supposed to insure a snug fit I guess. I never had done it that way before, but thought I would give it a try just to see how useful that technique is. What you're describing is the way I usually do it for any fuselage cutout: Lay the root rib on the fuse, trace around it, cut it out, sand to fit, slide the sheeted wing through the hole.
Yes, fuse sides will probably have to be spliced, wing centerline is the place to do it. Unless you have access to 6" wide balsa, which is usually spliced anyhow, may as well do it yourself. 4" may be barely enough for the stock side, but I'm increasing mine in height a little--1/4" or so to stay proportional to the increased wingspan..
Side note: The 1/32" basswood I mentioned earlier is too heavy for fuse sides. I think it would form very well, but only on a plane that can afford a little weight gain.
Still playing with the spinner.
--Ray.