Let me see if I can help you out.
I don’t care what head you use, if the rockers are NOT a offset, that port is good for maybe, MAYBE 370 inches before it becomes a choke.
Think about this. There isn’t an aftermarket head out there (again, with non offset rockers) that can compete with the W2 head. Not one. Not even close. And the W2 head was based on 340ish inches.
The engineers that developed that head weren’t idiots. And look at the lengths they went to make a competitive cylinder head.
And from personal experience I can tell you the W2 head, FULLY ported to square, with a 2.100 intake valve and a tunnel ram is about used up at 8000, maybe 8200. That is a fact.
Now drop those heads on 408 inches and watch them struggle.
I say it all the time. Architecture matters. You can look at some of the head available for SBC engines out there. To make real horsepower they are using heads that flow over 350 CFM at .500 lift and have a port size (and a really nice shape) that 25 years ago would make a BBC blush.
I think way too many don’t understand how head deficient these engines are. And the upshot of that is it takes a bunch more cam timing to get the same RPM (at whatever RPM that we are talking about) out of an engine that is induction (and valve diameter) limited.
The best remedy for that is reduce the stroke.