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

The factory ammeter wiring is questionable at best. They can barely survive going to the later 65A (square diode cooler back) alternator. In those systems total alternator output is going thru two 1/4" blade wire terminals in the bulkhead connector, twice before finding the battery on the other end. I would recommend any time you jump to a larger alternator to completely bypass the OE wire, leave it connected though, otherwise you loose power inside, and fab up a larger gauge wire directly to the starter relay then to the battery also. Instead of using the, or any Ammeter, use a voltage gauge. Anything above 13.1v engine running means it's charging. 14.0v is better but 15v or higher is bad; your boiling the juice out of your battery at that point.
Case in point, 100A aftermarket one wire alternator feeding:
Two 16" fans (20A each)
Electric water pump (8A)
Electric crankcase evacuation pump (30A)
MSD (?)
Fuel pump (?)
An additional multi-strand wire went directly to the battery (a.k.a. starter since the batt's are in the trunk). This car's favorite pass time was overheating, limiting its range severely. Heaven help me if I turned on the headlights!
I replaced the alternator with a 250A beasty (I have plans on EFI and a larger fuel system) replaced the add on wires with a 4 gauge cable in addition to the OEM wires. I ran everything I could turn on for a while and the overheating problem was gone. Voltage never drops below 14.0v. Besides everything else it has to keep two Optimas topped off.
Do I need all 250A?? Probably not all of it, but I'd rather have more than enough than come up short.

Hope this helps