Hi Nuno,
You can cut a “V” into the foam and lay the GPS cable below the surface. Then glue it in place with Beacon Foam-Tac. Perhaps you can also slightly lift the aft end of the wing assembly to raise the motor pod.
The 9oz Multistar 3s 4AH pack balanced the stock Bix3 perfectly. The Bix3 QuadPlane conversion uses a ZIPPY Flightmax 2800mAh 4S1P 30C Lipo pack to start which weighs about 10oz. This information is in post 1 and 4, respectively.
I have continued my Bix3 conversion thread here using Plane v3.8 firmware.
Cheers!