On my fathers engine, they dyno'd that with the smallish 2" chassis headers that he runs in the car, so you get a more realistic number. No fancy dyno headers were used that will usually add another 30-40 HP to an engine like this.
On a side note, they ended up using the stockish Edelbrock heads on it that was on the previous 455 that I built for him. Tony worked them heavily to flow 360cfm on a very small port. However they still hold back an engine of this size. Tony said it would be easy for this engine to top 1,000 hp with a set of aftermarket cast McCarthy RAV's or Tiger heads but that added another $5,000 to the cost, special headers, etc...It's a domino affect. So my father drew the line and said 700+ is good enough.