The header length calculations are just not that accurate. PipeMax can get you very close but still not 100%. Build it as short as possible with a few pipe diameters straight before the collector. Then build 3" extensions for testing on the dyno.
I built this for a 914-6 project some time ago, collectors are by Burns. The 3" extension clipped off 400 RPM at the top and was not a good trade off for the build.
Good luck.
PFM