That's what happens when A - you know what you're doing and B - you put decent heads on it...lol. The 4" arms respond the same way - but they need better (more port volume) heads to carry the power upstairs.
my heads flow 280-290 at 500-600...Thats pretty good flow if you ask me, but his are "Indybrock" Mine are just the RHS Indy heads. ANd I do see the theme of high lift cams making this kind of horsepower. That would help explain it. Also didn't see that it's a roller block. Either way nice job!