I'm installing a stereo system in my 72 coupe. Here's the thread on the C3 general board:

http://forums.corvetteforum.com/c3-g...o-install.html

The tech question is regarding my alternator. Will I need to upgrade it's size? My car is an A/C automatic, slow-poke 200hp SB.

 

Replied in your other thread about that amp. Though the manual doesn't state the potential current draw, being that it's a class D and probably 80% efficient, I'd say it has a potential current draw of approx 50amps. Take into account your car using 80-90% of the alternator's available supply and if you have a stock 40-60amp alt, then you can see you would be severely lacking in electrical support. This will be blatantly evident by your headlights and in particular your interior lights dimming to the beat of the music.

There are ways of stopping this, the easiest being to replace your alternator with one that produces more amperage than you'll need, ie: 200+. That will cost you about $250 not including the cost of wire and terminals to upgrade the cable supplying power to the battery.
Another is to replace your battery with one that has sufficient AH (Amp Hour) to feed the draw while being slowly replenished by the alternator. AGM (Absorbed Glass Mat) batteries are ideal for the demands of a sound system as they can be charged and discharged quickly.
Depending on your present battery, electrical draw and alternator, I'd recommend a 140+amp alternator and a 60AH AGM battery to support the cars normal draw, electric fans and sound system. Upgrading the main supply cables is essential too, as the larger alt and battery are pointless if they aren't being fed fast enough. The best route to go is to beef up the "Big 3" (Alt to battery, battery to chassis, chassis to engine/alt) with 4awg-0awg, then if dimming still occurs, change out the battery. If that doesn't fix it, then do the alt.

 

666WMD is giving good sound advice. You can also have your alternator reworked and upgraded to provide more amperage (you'll still need to upgrade your wiring though). Look for electrical shops that rebuild starters and alternators, see what they can do for you. I had mine reworked to 140Amps for $100.

 

My 82 uses much more amperage than yours plus I have electric fans so I stepped up to the $100 140 amp from summit. Stock was 85amp. Both are close in price and should be more than enough for your application.