For all practical purposes, most people anywhere would be fine with a 2 AWG big 3. However, a 200+ amp alt is not practical. 1/0 would be advised for such an install. The mindset that most of us have is to go all out the first time around. That way it's done for any future upgrades. There's no sense in doing it twice. With that being said, either 2 or 4 AWG would be a definite upgrade for the fuse box wire. The factory wire is 8 AWG and even that's almost good enough. Because you are more than likely not adding any components to the fuse box, there shouldn't be any reason whatsoever to upgrade that wiring.