Ive tested that in a couple cars, but came to the conclusion, that the over all header design, IE, the length & diameter of the primary tubes, their diameter and the collector length and diameter seem to have a greater effect, so short answer is I doubt youll see a difference,
and theres also the fact that collector design, and the cam timing and engine compression seem to have a noticeable influence on results, if you look over the KOOKS headers below it should be very obvious that rotating the 4 into 2 section of the collector 90 degrees pairs different cylinders which due to the un-evenly spaced cylinder firing order has, at least in theory a much greater potential effect, but I've yet to see any major advantage,,its generally the collector length, degree of back pressure , cam timing and engine compression and displacement that seems to effect results, and cams with tight LSA in the 104-108 range on the higher than about 10.5:1 compression engines seem to benefit most
its always Impressive, to see the amount of work required and the details of that work that was required to build custom components, especially engine and related components ,especially when you considered, that you need to work around the frame,suspension and engine to get headers to fit and still try to maintain semi-equal length tubes.
many guys would have tried to do that custom fabrication work and given up after a couple hours ,after realizing its not nearly as easy to do as it at first might appear to be, I think you did a really nice job for a first try!,Id point out that Ive seen many sets that looked far less well done, produced from supposed pro-muffler shops, whose main goal was making a profit , which is as I,m sure you know now an unlikely prospect once you start counting the time and materials required,to fabricate headers that fit a custom installation with suspension and frame components blocking the easy routes
if you get a chance, after those headers are installed, let us know your impression of both how the car sounds and how it runs and accelerates
you might find re-reading the links useful here
http://garage.grumpysperformance.com/index.php?threads/x-or-h-pipe.1503/
http://garage.grumpysperformance.com/index.php?threads/calculating-header-design.185/
http://garage.grumpysperformance.com/index.php?threads/building-custom-headers.961/
http://garage.grumpysperformance.com/index.php?threads/is-backpressure-hurting-your-combo.495/
http://garage.grumpysperformance.com/index.php?threads/x-or-h-pipe.1503/