It isn't solely about weight reduction, it is more about weight distribution and to a lesser degree, getting the battery away from engine heat
You need a battery box that is vented and the vent needs to exit the car- that is a hose run from the box and out through the trunk pan or drop off. If you will ever run it on a race course you need a battery cut off switch at the rear. Batteries will last longer if mounted with the wide side running across the car, but it is a relatively minor difference. If you don't have a roll cage, you can bolt right through the rear rail with 3/8ths threaded rod. A fuse in line is really the best way to wire it and if you wire it wrong it can damage your alternator when you switch off a disconnect. I did it a few years ago-there was a post floating around. Added a roll bar, so it is being relocated to yet a third location
I have a spare taylors aluminum battery box if you need one, I'll sell it at a fair price (just don't ask who it is fair to!)