These Headers are designed for Chevy Big block 396/427/454/507/572 engines. They are meant to either be forward mounted as part of a Turbocharger system or for off-road trucks, buggies or boats. The design will put the Headers collector flange into the factory firewall if mounted rearward. Where the primary tubes meet the collector is even with the back of the engine block. The collector extends back an additional 6.75?. make sure to take measurements before ordering this header.