The fusing option is easy.
Take the fuse allowance, in this case 120 amps.
Multiply it by the voltage, so assume 13.8v, 14v, whatever you like
Then, multiply it by the estimated amplifiers efficiency. I always assume 75% efficiency as a worst case, which si probably about right for the amplifiers.
At 13.8v, the amp could possibly be in the range of 1250 watts. I think that's a good guesstimate, I'd say, to be safe at least.