Looks good, but quite a few batteries will fit under the hood. I have a group 34(IIRC) Optima Red Top under the hood of my ST Focus and has served me really well.(Didnt have to customize anything, just a little elbow grease, lol!) Also have a powermaster(forget model now) in back tire well area and no big 3 and 1800rms on two tens doing 144.1 on TL and get very mild dimming. Can hardly tell. Mine is in a trunk as well.
Look forward to the end result and what you test at on the TL. Should be pretty decent. If you had a good electrical before, I imagine you would have had higher numbers as the hatches get quite a bit louder than the trunks.
Good luck!