What type of fuse for Cyrix-i 400A Combiner

I bought a Cyrix-i 12v 400A battery combiner for my boat. The engine can take up to 600 amps to start on a cold day, and the batteries are capable of 800 CCA. I have been looking at fuses to determine the correct 400A rate fuse to use. ANL or Type T. But when I look at the Time-current curves especially for the Type T I see that the a fuse rated at 400 amps can sustain 750 amps for a 1000 seconds (that was from a littlefuse data sheet). I was expecting to sustain the higher currents for around 15 seconds or so. What type of fuse have others used for this application? I am thinking that I should probably use a 250amp fuse, as generally the alternator charge current wont exceed 100 amps.

Fuse according to the ampacity of the cable being used; the purpose of the fuse or breaker is to protect the circuit, not the device.

Thank you. I have chosen a 500amp mega fuse. After a lot of reading it seems like the best choice for this application.