It is completely safe to install a (sealed) battery in the passenger compartment of a car. Car audio installers have been doing it for years. Now it is recommended that the battery be in a sealed compartment and have a vent at least 1.25" in diameter running to the outside of the the vehicle (per IASCA rules).

Just pick up some 0 gauge wire at your local car audio shop. It will most likely cost you about 4$ a foot but you should be able to negotiate price with them if you buy some other goodies

 

I have my aux battery in the back in a battery box strapped to the rear seatbelt bolts. Last week I installed a current limiting automatic charge relay designed for marine use: bluesea.com/products/7600. It limits the charging current at 60 amps. Pretty neat little unit, so far so good.