Installing those headers is a real PTA. The easiest way is to remove the engine, put the headers in the chassis, and then reinstall the engine. Some people remove the cylinder head and then slip the headers in place. You may be able to lift the engine enough to slip 2&3 in place without completely removing the engine, however. They're nice headers but a very tight fit. Have you talked to Centerline about this? They may have some better ideas.