If battery is in stock location then parts store "off the shelf" cables are OK size. This is especially true if you are going to a mini-starter which draws less current
If the battery cable goes from battery to starter relay, and then down to the large starter lug then the lower cable needs to be larger than 6. Use 4. Parts stores sell "starter" cables (eyelet to eyelet) which should be fine
You should not need TWO no8's through the firewall if you are eliminating the ammeter. A pair of 10 or a single 8 is way more than enough. REmember, if you run the alterntor to the starter relay, you have ELIMINATED the heavy charging current that used to go through the bulkhead (ammeter).
Be sure to fuse link or breaker the alternator wire, if the alternator shorts, there will be no protection.
