I have a Kodiak tank mounted on the swim step of my NR O/S. It is plumbed into its own pump and draws water from the same source as the wash down.
I have twin motors and an oval tank. I can not raise the starboard motor with the tank installed. I would use a round tank if I were to do it over. I don't think the back fish box would make a very good live bait tank. The fish like round corners. And usually those boxes are not very big.