[Al Skierkiewicz]

Please remember that the 120 amp circuit breaker can withstand a 600% over load for several seconds without tripping. Although the battery is capable of 600 amps for a short period of time, it is wise to design for well below that current. The excess capacity is used during the stalled motor condition which takes place anytime the motor is not moving but power is applied. (i.e. starting)
Also keep in mind that the breaker protects the wire not the load. A 40 amp breaker is needed to protect #10 or #12 wire (or larger). A larger capacity battery in the same physical size would be nice but I would not expect a huge increase. Maybe move from 18 AH to a 19 or 20 AH.