not to speak for jake, but I think he is talking 'rear wheel' hp (rwhp) whereas you are talking hp at the crank. assuming 15% loss through the drivetrain, he is making 350 to your 280.
also, these are peak numbers at 7000 rpm; his torque and hp curves are probably a lot fatter at all rpms - ie, more power sooner - as I presume he started with a 3.8 which are known for their grunt at lower rpms. of course, I have read that stock heads with bore increase tend to make for peaky performance, so expect some gains in the lower rpms as he optimises the output.
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