Assuming that the stock alternator was good for about... 60 amps??? And assuming that a C3 in stock form would suck all of those 60 amps, we have a starting point.
So, let's say that your main concern is your subs and Neon lights and NOT your 164hp L48 mill. You decide that you want to mount two 15" long travel subs in the back of your Vette. And yes, they'll fit, mounted at an angle.
And each of these subs are monsters and can handle a whopping 2,000 watts (not all that unlikely, I have three 15s in my room, purpose built and they can all handle 2,000 watts.) So now we need an amp that can put out 4,000 watts (RMS) at 13.8 volts that would be 4000/13.8 = 290... amps! Now we're going to go with an electric fan (not all that uncommon) and I think that Mark IV fan that everyone is in love with will suck ...40 amps??? So 60+290+40 = 390, throw some Neon bling under there and we're at 400amps and some OTT!
Admittedly, this would be one unusual C3. And I personally am running an aftermarket head unit and two electric fans on a stock alternator. In my highschool days before I cared about weight I actually had a 500 watt amp and a 12" sub in there. But still on the stock alternator.
With 140 amp alternators being fairly common, with some quick rough math once again, you'd need at least an... (140-60 = 80x13.8 = about 1100) 1100 watt RMS amp to suck up all of that, at least. And seeing as how most aftermarket amps have bullshit ratings (rated at MAX power which is ususally twice as high as the RMS output, or purely a bullshit number) you'd realistically need more than a "2200 watt amp
" to suck up all that just one high output alternator could supply.
And yeah, clocking has nothing to do with dual alternators.