Motoman, It sounds like you are doing (or have already done) almost exactly what I am preparing to do. If finished, did you have any issues or have any suggestions for mounting the Fuse/Relay box above the driver seat?
I thought about each individual part of this mod for about 9 or 10 days before deciding on that location. That night I read your post.
I have my second battery installed, but still waiting on the battery isolator to arrive so I can continue with the electrical. I don't know exactly what your lights will pull, but my front light bar is 270 watts, and I am told the draw works out to be about 10 amps. Looking at the pigtail that comes on the light, it looks to be about a 14 or even a 16 gauge wire. If it indeed pulls 10 amps, that wire is going to get very warm, so I am guessing it to actually pull less than 5 amps. Going to do a temporary hook up tomorrow and actually measure the amperage. Will post that later.
Calculating for my circuit (roughly 500 watts total if everything is on at the same time), lights, USB power outlets, Stereo and a little extra I may need for something else I may want to add down the road, I am planning on using a 6 or 8 gauge wire to feed the block and then just fuse it at a lower point until I need more power to that block. If you haven't already finished your project, I would assume you could go with a lighter wire, possibly even a 10 gauge. If you have a multi-meter, you could do like I am going to do and do a temporary hook up one of your lights to a power source and measure the draw. That will tell you your minimum requirements.