Stock alternator up to the task? If not, how many Amps will I need?

BillGrissom said

You will power the fans via a relay, so current stays in the engine compartment and won't stress your bulkhead and ammeter any more than currently. Realize that your dash ammeter won't show the current consumed by the fans, just as it doesn't show current used by other under-hood loads (starter, horn, add-ons), so with fans running the ammeter might show "C" when actually the battery is discharging.

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Keep in mind this all depends on where you tap in for your power source. If you tap in between the alternator and the bulkhead - yes, the bulkhead connections, as well as the ammeter, won't see the draw. But, if on the other side of the harness, between the bulkhead and the battery it will see it and the load through the OE wiring will be there.
Most folks tap in at the starter relay, or the battery itself, as it provides an accessible and easy point to add a hot source lead (just like the factory did with a fusible link). Either of these will draw through the ammeter circuit bulkhead connections. The added draw translates to heat through small gauge wires and weak, at best, connections. That's why I add a heavier wire directly from the alternator to the battery and switch over to a voltmeter.