since you are using CWM recovery you can modify the OTA package without worrying the signature issue.
1) extract the zip package to a folder.
2) open META-INF/com/google/android/
3) use any text editor you have to edit the updater-script file.
4) remove the assert statement
5) repack the folder into a zip package
then flash it with CWM recovery, and it will probably work
didn't see you following posts. nvm
Samsung Nexus S (Unlocked)
Android: 4.0.4 (IMM76D)
Kernel: 3.0.8-g6656123 (Stock kernel)
Recovery: Clockworkmod Recovery 22.214.171.124 (Touch)
Motorola Xoom Verizon 4G (Locked)
Baseband: CDMA_N_03.1A.5DP LTEDC_U_05.19.00
Android: 3.2.6 (HTK75H)
Kernel: 126.96.36.199-g6963abe (Stock kernel)
Recovery: Stock Recovery