Also, when looking for an amp, look at the RMS power rating. The 800 watts in this case is most likely the max power rating. I also have a 72 super, and am just installing an Alpine V12 Amp with 600 watts max, 400 watts RMS. If I were you, and looking for an amp to run off a stock alternator, I'd play it safe, and try to stick to something below 500 watts RMS. You can also find out how many amps your Alternator is, and ask your local audio shop whether or not that amp will work. Those guys are allways pretty smart when it comes to taht kind of stuff.
Many people will try to tell you that a Capacitor will fix your problem if you have too big of an amp, or a cap will let you run a bigger amp. This is in fact false. If you want a big system, you need to get a big alternator, not a cap. An amp with around 400 watts RMS will pound a sub fine in a 72' super.
I was going through the same thing you are, and deciding how big of an amp will work. I posted a forum on shop talk forums. It has lots of helpful tips. I hope it'll help you, and hope it makes sense, you can view it here....
Will I have enough power-do I need to uprade the alternator?