Hoe long is this headpipe? A shorter pipe can handle a smaller ID to keep velocity and scavenging up even out of a log manifold. The longer smaller id runs ones will hurt performance. If your just mocking something up to get fabricated, use the flexible stuff you get in 12" -18" lengths at autozone and create the shape needed and take it to a muffler shop to fab it up. Easier than driving it there at 4 in the morning open manifold to avoid the cops for a noise complaint or possible burning an exhaust valve in the process.