Gauge of wire depends on 2 things. 1) how many amps the cable needs to carry and 2) how long the run is.
85-105 amp alternator needs 8 ga cable up to 7 feet to flow at its full potential. After that it needs 6 ga cable up to 10 feet and 4 ga for any reasonable length beyond that.
Regardless, I would run a 3 wire set up to protect from over charging. I am sure you are aware how easy they are to wire up, but here is a page that does an awesome job of summing it up:
http://bob_skelly.home.comcast.net/~...ternator1.html