Currently in my battery case
Seriously, both Ben and I are running the shorty pack down the center line of the car with very little modification to the car. All we have done is tweak the battery hold down clamp to use thumb screws and shortened it to give better access to the shorty connections.
At Kidderminster national I ran the pack in both the forward and back positions depending on track conditions (ie grip levels in the wet) and the car responded very well to this change. Also running the pack down the center means no modification to the side pods which would drastically effect chassis flex.