The two 8 gauge wires will be fine. try to keep the distribution block as close to your amps as possible. The shorter you run the 8 ga. wire the better. I think your fuse is fine, It should blow at the battery first (hopefully) and not at the amp which is scary, considering the amps are hundreds of dollars and the fuse is 2 dollars. The ground wires can just be run to the same mounting point, there is no need to run them through a seperate distribution box, unless it is for looks.