Does the game decide where in the creature's body it puts all the squishy organ stuff or can you manually decide you, say, want to put his brain in the tail?
I believe there are two "types" of parts for all creatures -- parts you can control the directly placement and parts you can't. Most parts you can directly control the placement, such as legs, arms, eyes, mouth, etc. But some items you can't directly control, such as the backbone (you can control the size of the backbone, but not the placement, it needs to be in the back).
However, the only item I saw which "needed" to be in a certain place was the backbone. Everything else seemed to be fair game. Since dinos had brains in their tails I would imagine that it might be an option to place a brain in a different part of the body.
However (again) if the brain placement doesn't directly figure into the gameplay (i.e. the creature will behave the exact same if the brain is in the head or the tail) then I don't think you'll get an option for brain placement.