My goal is actually to come up with some sort of compromise.
On the starter side, I've upgraded to a gear reduced starter on other vehicles, and they just don't sound the same as the originals.
On the cable side, I think I can get away with 4 or 2 gauge and keep it looking original. I agree that anything thicker won't look right.
So given the current draw and lengths, one idea is:
Move from a 6 gauge to a 4 or 2 gauge cable for the short ~9" positive battery to solenoid
Move to a 0 or 00 cable from the ~32" solenoid to starter cable which is much less visible
Move from a 6 gauge to a 4 or 2 gauge cable for the battery negative and then after it loops around the side of the battery ~6" on, splice it to a 0 or 00 cable for the remaining 30" where it's not visible.
That should be "almost" electrically equivalent to all 0 or 00 gauge cable.