Dont write the nand image back to the console with nand-x , do it via xell . The bad blocks wont effect the xell install as they are after the xell address so get that installed and get your key ,
make the image as normal using multi builder v0.2 i`ve had issues with 0.3. Place the .elf file and the image file on a usb stick , put the stick into the xbox and boot xell . From there it will take over and install the nand image re-map the bad blocks for you , you dont need to do anything . i`ve done this a good few times now and it works 100% that way and saves you muking about trying to remap them yourself .