Cable Size, Amperage for my system

2002 Yukon 5.3 not sure what size alternator but schucks.com says standard is 105amp.

MTX 6500d class d, mono block will be running 2ohm load. 12.5v 2ohm load 500x1, 14.4v 2ohm load 775x1.

Both amps have 75 total amp fuse ratings (3x25).

Now I looked at the Recommended Power and Ground Cable Sizes page here but I didn't feel it answered my questions very well.

based on the info there 500/775watts = 80/125amps yet the 6500d will do 500/775watts with only 75amps of fuses.

Would 4 gauge power be big enough for a 12 foot run? And would my stock alternator be big enough? I have 1 farad cap also. I do like to turn it up loud but not for extended periods at a time.

I know the 6500 is class d and more efficient I think its 60 maybe even 70% thought I read that somewhere, while the 302 is maybe only 50%.

Well guess I will just see after its install what kind of voltage levels I see with and without the cap

No sure if I need to upgrade the alternator one. I think its 8 gauge wire that runs about 12 -18" to a junction box that ties in with the cable running to the starter. From the junction box to the battery is like another 2 feet of 4 gauge wire. Should run 4 gauge straight from the battery to the alternator. That would mean having 4 wires coming from the positive post. (starter, alternator, amps, fuse box under hood)

So let me get this straight. There is two wires coming off the negative post. One goes to the chassis and the other goes to the engine. Those are the other 2 wires I need to upgrade? From my understanding there should be one from the engine to the chassis as well?