I would suggest using at least 2 awg wire but 0 awg would be better. As for a battery in the trunk, I would look into Stinger or Odyssey batteries. I have a stinger in several cars and much prefer them to the Optima (For the record, I ran a car stereo store for the 17 years and moved alot of batteries around as well as sold just about everything out there). The stinger comes with a steel mounting box to help hold it and they come in two flavors- Yellow= cheaper, not as good or red= best quality and longest life. Check out their link at
http://stingerelectronics.com/...r.asp
Dont forget to protect your car with some type of breaker at the battery and ground the battery to the frame underneath the car. This is best done by drilling a whole and running the ground cable from the battery to the underside of the car. Next, use a wire wheel in a drill to clean the metal where you are going to ground it.Through bolt the connector to the clean steel and then cover with spray on undercoating (3M works well).
As for running the cable up front, under the car means that you have to worry about every future moron who might jack up your car, crushing and shorting out the wire. The frame rail is a good idea but a lot of on your back work. I generally stay in the car, after all, Nissan didn't run much wiring under the car. Now, getting through the firewall is a challenge so I would go through the fender and follow the factory harness behind the fender liner. Then buy a nice distribution block from a car stereo store to mount up front and your done. Will probablly take a day or two depending on how many helpers and how many beers. Oh, and how good of a job you want to do....
Good luck
Scott