Your NAND-x is perfect for writing the xell image to your xbox (image_00000000.ecc), but when it comes to the final ggbuild image it can be easier to update the NAND using a software nand flasher. Rawflash let's you update your NAND from within xell, by putting your new NAND image onto a USB and then booting xell.
You need to get xell installed on your console. Then you need to grab multi builder 0.3, which includes rawflash v3 in the data directory. Rawflash is actually just a file called 'xenon.elf' that you put on a USB key, along with your ggbuild custom NAND image (you have to rename your Nandimage.bin to be 'nandflash.bin'). Then you boot your xell console with USB inserted, and it will tell you it is updating your NAND. Then it will hopefully work.