Need to be rated at least for the alternator output, that is 100A but not more than cable ampacity. Typo? 125A? I think if this fuse fries so will alternator diodes. Don't want to trip it. AFAIK 6AWG ampacity is 102A so not much margin here.
Oh well, 4 AWG it is... Guess I was trying to fool myself into using 6AWG and terminals that I had.
Sergei