Do you know what the engine specs are? What heads it has, cam specs, things like that. I hate to even think it but it seems every Chevy guy is sure their engine is making 500+ hp. It sounds like the previous owner fell into that group.
Oval port heads on a well built 454 can make as much as 550 hp but not every 454 with oval port heads makes that! I just finished a low compression 454 that made 402 hp at 4700 rpm; plugging those numbers into the calculator says that this engine in your Firebird should run almost 10 mph faster (the final speed tells you fairly accurately how much hp you have), while 550 hp would be another 8-10 mph faster again.
The time it takes to get from the 330 foot mark to the 660 foot mark and then to the end of the quarter mile can be used to calculate with good accuracy what the engine is making as long as you know the vehicle weight and gear ratios in the transmission and rear end. I made some guesses based on what you said to get the 300 hp figure.
