I'm going to be redoing the negative (ground) to top post, the stock obviously won't reach up there. I'm going to strap the close body ground with 1/0 gauge that I already have, could someone tell me where the other leads to? I see it going down towards the engine but I don't know exactly where. I'll be replacing this with 4-gauge.

I know there is the big black one that goes from the battery to the engine block just behind the A/C compressor. Then a small one that goes to the body next to the battery. Am I correct to assume by your post that you have 3 black wires going to negative?

I think I see two, the larger going down towards engine, smaller hitting a body ground behind the battery. Is the one behind the a/c compressor easy to access from underneath? I'd be replacing this.

From underneath, yes its easier. You would have to remove the splash shield first.

Would about 5 feet of cable be enough or?

That should be plenty.