I am afraid it was not my idea. I was looking for mock up ideas and tripped across this... https://www.youtube.com/watch?v=9s1-AZcPWAA
turns out http://www.stainlessheaders.com/ is a sister company of GP Headers- http://www.gpheaders.com/. You rent the mock up header flanges, merge collectors, a dozen swivel joints, and some PVC angle fittings from GP Headers ($600 deposit). You can then weld up a fixture to hold the mock up, and use header pieces purchased from stainlessheaders.com or other vendors, and return the mock up pieces to GP Headers to get you deposit back, or ship the mock ups to GP Headers to have them fabricate the headers for you...