The bottom links below are some pretty good images for you to look over to help know if you can install the gateway system.
Being you have a an 2005 350 MPI engine model, there is a small possibility that your engine may not be compatible due to "I believe" somewhere right in the 2005 - 06 year timeframe is when merc was making changes to the engine wiring harness to be more smart craft compatible on the small block engines....Could be wrong though.
Two things you can try doing is to see if your engine has the CAN wire harness connector similar to the ones in the photos linked below even though those are for big block 8.1L's engines, the plug harness should look close to the same...The other would be to call or email mercruiser customer support with your engine serial number to see if they can tell you if it would be able to work on your engine or not.
https://www.mercurymarine.com/en/us/...mer-assistance
According to one of Merc's Q & A websites, it may be compatible but if you can't find the CAN connector, then a call may still be necessary.
Question: 'I would like to add SmartCraft instrumentation to my boat. Is my MerCruiser engine compatible? Can I install this myself?
Answer: 'All Mercruiser 4.3 MPI, 5.0 MPI, 350 Mag MPI, and MX6.2 MPI Sterndrive, Inboard, and Tow Sports models with serial number 0M300000 and higher and all MerCruiser 496 Mag and 8.1s Sterndrive and Inboard models are compatible with SmartCraft instrumentation systems. Mercury Marine recommends installation of SmartCraft systems be performed by your Mercury Marine dealer.
The thing with that quote that makes it questionable is smart craft started with some outboards in 2001 a long with the 496/8.1L's in 2001 then added other engine models over the years there after...That's the reason you may have to call should you not be able to find the CAN plug on your engine.
http://s250.photobucket.com/user/rlu...aa7e0.jpg.html
http://clubsearay.com/showthread.php...120#post608120
http://www.bing.com/search?q=mercrui...1E41962AF04846
Hope it's there, Let us know and good luck.