What size power wire do I need?
 

I think I can get away with 1/0 gauge, but I want to make sure.

My amps are:

Diamond D5 1200.1 Class D - Will be running at 1 ohm producing 1200w RMS
2 Diamond D3 600.4 - One of them is bridged 300x2RMS, the other running 75Wx4RMS

The distance from the batteries to the amps will be around 6'. Is 1/0 Gauge for the power and ground enough or should I go up to a 2/0?

I've found it difficult to find distribution blocks for 2/0.

Any idea of what fuse size I should be running with this? Is a 250 amp breaker enough?

You definitely don't need 00ga.

I'd do a quality 0ga wire, 300amp breaker at the battery>fused distribution block with (1) 0ga input and (3) 4ga outputs, 100amp fuses for the 600.4's and 125amp fuse for the 1200.1

You are right in saying that it is very difficult to find a dist block that will accommodate wire larger than 1/0.

You can always run two 0 gauge power lines to the dist block. Take a look at this dist block - it will allow you to do what I just mentioned

http://www.darvex.com/store/pc/Sting...ock-26p796.htm

And that is a pretty good deal on one as well.