Just installed my 546s with the transom mounted transducer. I installed it on the starboard side between the engine and the factory mounted speed item. I read that it should be mounted based on prop rotation, but I am not sure how critical that is since the transducer is in the water before the prop. Let me see if I took a picture.
It is just to the right, about 4-6 inches. Garmin gives you a template for the mounting location. I ran the cable up to the same height as the factory speedo cable. Garmin gives you a nice little black plastic cover and I filled everything with 4200. Even dipped the screws in it.
Not going to apply to your application, but here is the gps mounted in the helm.
