The cabane struts are slightly too long--even if you are careful to install the right struts in the right location. My solution was to install the interplane struts first, with the aid of jigs. Once the struts were set, I glued on the top wing, and let that set hard. I then inverted the model, and test fitted the cabane struts. A little careful trimming and they clicked right into place, leaving a perfectly flat top wing.
Here is a pic of the jig--made from thin card stock, they are handed, so 2 are needed. Worked great, easy to make, and aligned the struts perfectly.
Here is a pic of the finished model:
Good luck! It's a nerve wracking kit, but if I could build it, anyone can!