note that step 39 ask you to put one more small 6*6 piece in the front.

one more rule when you build a kit
when you have to cut several parts in balsa spars, CUT LARGE PIECES FIRST. otherwise theres is a risk that you won't have enough long sticks remaining to make your long parts.
In the laser 3D, there is more than enough wood in the box, but if one day you build a sig kadet senior, you will understand what I mean