The majority of the weight in an s30, or any z for that matter is not over the rear. They are slightly front biased. I feel that most people who concentrate on moving weight rearward, unless dragracing, are mostly just adding weight to the car. Those are some pretty big wires you have to run all the way up to your engine and starter. Me, I'll get the smallest lightest battery I can afford and mount right next to the starter. Yes it is still in front of the firewall, but it weights less than a third of the original battery and is much, much lower. Just my opinion and I'm in no way an expert... come to think of it the o.p. wanted to know what battery, not where so back to the topic.