I would use one of the adjustable transom mounts. Reason is the ease of adjustability. With the steps you are going to have to play with the depth of the pickup. My through hull pickups, which were done by DCB, are on the bottom of the boat. Fore of the inlet is trenched to allow water in. This would be a lot of work trial and error. Gel coating necessary. When I turn left or right there is a definite difference in water pressure.
It would be worth a call to Nordic to see where they would mount the pickup. If you put the pickup on either side of the pad it will have to be deeper. Worst case if you transom mount and don't like it with testing, it will be much easier to move with the motor in. Eric