This doesn't surprise me at all.
I recently put my car on the chassis dyno to optimize it and see the difference between the 600 v/s Holley and the 700cfm Double Pumper I also have.
It's a 354 poly, 727 and made 341hp at the wheels with the 700 cfm and 334hp at the wheels with the 600. It was a filthy hot day, 90° by 11am and nearly 100° by lunchtime.
The dyno guy was very impressed - said it was a really stout engine, the factory 4 barrel manifold is holding it back in his opinion.
For reference the stock 428 Cobra Jet Mustang he ran on there earlier that morning made 290hp. That was an auto too.
I was surprised the power started tailing off by 4,500rpm, it's only a small-ish cam coupled with the factory intake. I do rev it 5,500 rpm though and it still pulls.
It's a 318 poly with a 3.58" Scat forged crank, Scat rods, Ross pistons 0.065 over, 10.8:1 compression. Poly heads with a little bit of porting, 1.94"/1.60" valves, Schneider solid lifter cam 264F (220° at .050, 0.450 lift). Topped off with a factory cast iron intake and an 1850 Holley 600. It's got headers, stepped from 1 5/8 to 1 3/4, 2.5" dual system.
Fancy that, an old poly small block making 50 hp more than a 428 Cobra Jet. Okay, it's not quite a fair comparison as mine is modified and the Cobra Jet was stock but still...
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