I have a 2001 5.7 GXi-A w/ a MEFI 3 ECM. Suggest that you look into the Fox Marine NEMA 2K gateway - it's like magic! If you don't already have a NEMA 2K backbone on your boat, you'll need to install it ... <$100. Then you hook up the Fox Gateway to the ECM diagnostic port (OBD) then connect to the NEMA 2K drop and if by magic ALL of the MEFI 3 parameters are available for display on your NEMA 2K chart plotter. If you don't have a NEMA 2K chart plotter - no worries. The gateway has a Bluetooth transmitter that interfaces easily to the Fox Marine app on your phone. All parameters are available real-time on your phone.
It's a wonderful system for the older MEFI engines - about $300.