I'm just saying they have to be decent to carry the power that high. I have been north of 280 @ .600" on emissions heads with a 2.02" valve, so I know the possibilities when in the right hands. I'm thinking they had to be at least 240 cfm.
I guessed 420 based on thinking 240 cfm, which to me is "some porting". With 450+ horse I'd guess they are between 250-260cfm. Which is a little more than "some porting" in my books!
Either way, makes great power. There's a shop north of here that's charging over 10G for a 408 stroker that makes 10 horse less than your thrown together 340. So you're definitely doing something right!