a. graham bell has been a respected authority on exhaust design for a long, long time. i used his many books on the subject to design 2 stroke schnurle principle engine exhausts 25 years ago. i have a couple of his hard bound books that have been out of print for 25 years. he had a really concise and well written chapter on exhaust tuning in 'the design and tuning of the schnurle principle 2 stroke engine' which was what turned me on to bell more than 30 years ago. see if your library has anything by mike moss, too. he wrote more from a 4 stroke perspective but a lot of his writing was taken from graham bell's work.
if anything, bell's books are written from the stand point that anyone who attempts to use his data is already knowledgeable in all the intricacies of sectioning pipe and bends, welding, flow dynamics, and lots of metal working basics. because of all that, building pipes is as much voodoo and artistry as it is science and mathematical calculations.
believe me when i tell you that i used bell's ideas and math to build expansion chambers incorporating some complicated forward and reverse cone designs and dimensions that tuned up true to his math. practical applications of exhaust systems based on formulae gleaned from his book have worked out great for me but i had to be able to translate the math into real steel.
having been educated in electrical engineering, i have always used math and equations whose functions are applicable in both electrical and mechanical engineering. that usually means that i translate rpm into frequency and solve the mathematic problems from that angle. it just makes it easier for me to wrap my head around.
you can find a lot of good fabrication guidelines in the machinist's bible and the bosch automotive engineering handbook. i can also think of 2 or 3 welding digests that have the specifics needed to fabricate cone sections, tapered bends, and tricks on pipe sectioning.
a couple texts for 100 and 200 series flow dynamics come to mind, too. google laminar flow and read that until it clicks for you.
there's some good data and applications that can be found in texts from the sheet metal tradesmen, especially where round duct work is being featured.
i used to build some pretty ugly, quick and dirty stuff to see how it worked out. when i found something that worked i would clean it up and refine it. a lot of prototype work i turned out worked great but was uglier than sin. i have the highest respect for guys like jardamuth who take the craft to the point of being fine art, beautiful to behold along with solid engineering and advanced design.
bolt that header on and give it a whirl.
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but I did find another book from Graham Bell that goes deeper into these subjects. I only took a global look, but will an update after I fully go through it.
The primaries on the VW-header seem to be about the same ID, but the secondaries have greater OD's. I hope I can still use the Y-pieces though.