Behind wiper cowl, over to passenger side, behind fender and through passenger kick panel, under front/rear door sills on passenger side, into trunk. One of these days, I'll make a thread for relocating battery. Don't forget, install a fuse CLOSE to the battery!
Can't believe the stereo upgrades and you haven't touched wiring yet. I highly suggest putting your optima in the trunk. But where I used 2 gauge, you should go with 0 gauge. And I do I think it would solve your voltage demand problems, but ask a stereo expert for their opinion on that.