I used to have a 600+ hp '55 Chevy that ran 10.57 secs @135 mph at the strip. To be honest it was a frighteningly quick car, but was a right pain in arse to drive on the street. You're constantly trying to tame the thing & on a long journey, it became very tiresome. Pull away from a standstill & the tyres would try to break loose, pull out of a junction, the tyres would spin up, cruise at 70mph & give it some throttle & the tyres would just break loose. It sounds like good fun, but the thing's constantly trying to kill you.
I'd say mid 400 hp is a nice figure for a street car that would be used regularly. You say you thought an auto would be ok with more power, the down side is, is that you loose a certain amount of control with an auto. To get the power to work you'd probably need a looser converter, my '55 had a 3500 stall speed, which means the car will barely move until you hit those sort of revs. At 3500 revs you'd probably be trying to put down around the 450+ hp mark on a 700hp engine. So, as you can imagine, try to pull from a standstill using all of the power you have in your Firebird at the moment & see what happens. Wheelspin!!
Now imagine doing that everytime you move froma standstill. At least with a manual trans you can slip the clutch to get rolling.
As I said previously, it's your car & you should do what
you feel is right. Having been there, done that I wouldn't even want to entertain it in a street car. Is there a reason as to why you want 700 odd hp?
cheers...Nige
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That sort of power on the street will make the car a nightmare to use & could become tiresome after a while, but hey, each to his own 
As others have said, make sure the chassis can handle the power first 