Ulf, your doing a great job. My main concern when sheeting the fuse was keeping the thing straight. There is a lot of force required to bend everything together even in a soggy state so you must have the structure attached to the board and bend each side at the same time to cancel out the forces acting on each side of the plane. I started at the saddle and pinned along the crutch toward the tail, then turned around and pinned toward the nose. next I forced the sides together just aft of the cockpit area and while holding it in one hand, tack glued it in place with thin CA. I repeated this toward the tail, always pulling on each side with equal force. You will probably want to trim away excess sheeting as you go so you can see where your tacking. I also felt better checking eyeballing the fuse from the tail periodically to make sure I wasn't building a bannana!

In regard to 1/8" sheet, if you have light stuff it's probably no biggy. Much of mine was kinda heavy and stiff so I really had a tough time with it. My fuse is built like a tank! Let me know how that Titebond III holds up in the soak, I'm running low on II and will switch if your happy. Regarding spars, again, my wood was on the heavy side so I put the densest balsa on the lower spar and the less dense on the upper. (Airplanes pull more positive G's than negative during their service life.) Instead of spruce, you might put some 1/16 shear webs on the back side of each main spar pair with the grain vertical. That would be lighter, much stiffer, and possibly stronger than spruce substitution. One place that should be strong is the stabilizer leading and trailing edge. At high speed soft balsa can allow flutter to tear/break it off. This occurred on an upsized 2/60 version of this same plan a couple seasons ago.
Mike.