I had a lot of difficulty with alternators in my street/strip Mustang. I switched to a one wire from East Coast Auto Electric and installed/wired it by their instructions and it did the trick. Tossed the wire to the starter relay and ran it directly to the battery in the trunk. Used a Moroso 4 pole cutoff switch and it kills everything.
Wire gauge is based on two things: amps carried and length. There are charts online.
Link:
https://www.tessco.com/yts/industry/...get_wired.html
I used 4 gauge to run from the trunk of my Mustang to the alternator. It was the best I could do.
NAPA has bulk auto wire in battery sizes.
Dale