3/4 sI have mechman elite 370a.   Fuses ? running 3 lines ones of 2/0 to back xs d3100 fuses? To crescendo bc 6k  I'm gonna double up each power and ground need to know its bugging me crescendo says 250a fuse per power line so it would be 6 250a fuses total between back battery going to the amp anybody know ????? Thank you in advance !!! All 2/0 copper welding cable heavy 450a 600v ratings front to back !!!! And do you think 2 skar vxf 12 can handle that 3/4s of that power lol

Typically you're just fusing the wire to protect from a short to ground (cable punctured or comes loose in the car). Not sure if its different in these super high power situations. 

Depends on what you want to fuse for.  Are you fusing to prevent overcurrent supply to the amplifiers?  Are you fusing to prevent a fire in case of a short?  Are you fusing due to class rules?

To get most safe rms watts from power and to prevent fire !!!

Then fuse to the current rating of the cable.