Need help figuring out electrical

[bmxmon]

Hey guys, im planning on re-wiring my mustang this winter, along with the dozen or so other things I'll be doing. Anyway, along with the stock electrical equipment, I plan on running other things, such as an electric fan, fuel pump and a stereo down the road. I plan on using a universial fuse block and doing all my own wiring.
One thing I'm confused about is how is the fuse box powered? Looking through wiring diagrams, it seems like it comes from the starter solenoid, (one wiring for constant on, one for key on correct?) but it seems crazy to me to have only one wire having all the power for the fuse box going through it. If it is this way, should this be a larger wire? Does anybody know of a site which shows recommendations on wire gauge to amps?
Another thing that complicates it is the fact that I want to hide the wiring as much as possible, so I will probably mount the starter solenoid under the car, along the framerail, and move the battery to the trunk.
Im not an electrician, in fact I hate electrical. Its just I will have the car torn down to the point where I feel it would be dumb not to do this now. Any comments would be great, along with info/suggestions. Thanks guys.

[bmxmon]

Here is a site with amp to wire gauge size, and wire distance: Wire Gauge Tables