Heat and beat is the only answer! And I have built a total of two sets in my life as a welder and fabricator. Translated I have better than average skill. Ask me if I will ever build another set? Not only no but heck no. First set was for a Pontiac set of heads, fabricated a three pipe set for a V8 to eliminate center leak. Next set for for a Porsche race car competing in D production SCCA. I was on pit crew so did not really have a choice. Fabricating headers is a tuff gig period! Good luck!
