header: 3: A pipe that serves as a central connection for two or more smaller pipes.
In the case of 4-2-1 headers there are two headers serving as a central connection (4-2) and another one serving as a final connection (2-1) in which case it actually makes an exhaust manifold composed od tree headers .

By the way TX rob the only thing I was really wondering if the 02 sensor is removable in one piece and if there was any other parts to be taken out in order to do this swap