Headers tube diameter.
At 410 inches they need to be 1.875 at 500 hp. As it gets closer to 600 you need 2 inch and a 4 inch collector.
BTW the 1.875 headers should have a 3.5 collector. And both will probably need 18-20 inch long collectors.
I should add that at 340 inches and 7000 RPM I am using 1.875 headers with a 3.5 inch collector.