You should be okay with your 8 guage wire and 2 16 ga. fusible links in parallel like mine is hooked up. The added "charging wire" also doubles the input to your fusebox by having both the red and black wire now sharing the car's load. The main thing is the charging circuit now bypassing the bulkhead connectors .
If you go by tha chart that Bullit linked, it says that you should measure total distance to the device then back. For example; it says that for 70 amps max, for going from the alternator to the battery then back (about 15') would require a 4 gauge wire. Some of the charge wires in the newer cars are big, and you'll never pull a full 100 amps, but a 4 ga would be safe. That's a little too big for me.
It also says that fusible links throw "sparks and smoke" when they blow, so I wouldn't wrap them up inside a harness. GM always put theirs in "open air" on their firewalls too, like mopar did.
