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 to the n2k network, along with other things like fuel and holding tank (via a seperate n2k module), which are then all visible on my axiom 9+. This is the same for most newer MFDs. For our boat, I pulled the speaker at the helm and was able to fish a long n2k backbone cable from the dash to the engine bay. In the engine bay I have a n2k 4-way tee (2 for engine gateways, one for fuel/holding tank, and the other for n2k gps antenna, which was easier to connect there, coming down from the arch, than routing back to helm. In the dash I have another 4-way tee for the axiom itself, my vhf, and my fusion radio.
The company Yacht Devices has gateways, too, but those turned out to work only with newer 2004/2005+ ECMs, but they have other accesory modules that might be useful (that's where I got the fuel / holding tank adapter).
Fox Marine has an app that allows you to view a lot more engine data than the MFD will display, and it works pretty well - I think you can view it from a tablet, too. But if you already have the axiom, you just need a n2k network and the engine gateways.
