I would think that the length of your custom header would be critical. 1/4" too long or too short and all advantage of having a new exhaust could be lost. 1/2" either way and all advantage of having a tuned pipe could be lost.

With tuned pipes, length is everything. They are like a trombone, where their length changes their "tune" and what RPM range they are optimizing. You can easily push their tune outside of the rpm range of your vehicle and lose any power gain you get from them.

Just a heads-up for you guys with custom manifolds.