I would consider getting a piece of starboard to mount first to allow for customizing location after the fact. See that on a lot of boats - seems like a good idea.
I just mounted mine tonight. I mounted it pretty much right next to the center drain (right or wrong...) I bought a Hummingbird HDR650. And just as an FYI... the included hardware sucks. I broke off two screw heads after pre-drilling the holes the way the directions say.