I had a Q-Jet on my 305....Like yours mine was fine for normal drivin but when I floored it after a while the engine would choke out. And I would have to let off for a little and then slowly get back on it. Eventually my top speed was reduced because I couldnt take it any higher (and that was when I was driving normally) My solution was to get a new Carb and Intake. Now I dont know if thats the solution for you but it sounded like both Camaros were doing the same thing...

 

There are many reasons this could happen. The vacuum would be first look on the card are there any open slots where a hose would go? Also did you check the timing that is another factor? Tells us more about the setup all I got was truck motor and carb. Did you have this issue before the carb was rebuilt? What is the CFM of the carb?