600Wrms/14.4v=41.6A
for simple AB amp, with efficency of 50%, u need 93.2A, this is peak stuff, ur battery will have much higher crank Amp ratings, and if crank rating == how fast a battery can provide Amps, u shouldnt run outof power.
im not sure of average efficency of class D.
in any case, its better to add capacitors than replacing altenater.