As far as i know the APIM itself cannot be upgraded. You either get nav or non-nav. After using Sync 3 with Android auto and Waze you'll realize you don't need factory nav.
The only thing we got when we upgraded my wife's system was the screen with the APIM attached. It looks like this includes the entire infotainment system. You might check with your dealer to see if they can give you more information about whether or not this equipment can be installed in your car if you plan to use it all. Whatever you do you'll need the faceplate that goes with the larger touchscreen. This will probably include all the HVAC controls. I've never seen a GT 350, so I can't say whether the faceplate and HVAC controls are the same on those cars or not. I also can't tell you whether the connectors on your car will have to be changed.