2003 as well.  The 2003 5.7 GXI-E engines were, in my case, MEFI-4.  You'll have to verify what what version of MEFI your ECM is, although I'm not sure if the 8.1mercs even used MEFI.  But otherwise, I converted all systems on my boat to nmea 2000.  Each engine has a gateway, which are connected...