They say it is 3/8 but I'm not sure about the primary tube thickness, as for separating the flange, I think I could cut it after building the header?

http://www.summitracing.com/parts/HOK-11654HKR/

Allen Here's a novel idea, let's not throw a bunch of parts at the car hoping it will fix the problem and instead spend a little time diagnosing it first. Life was a little easier when I was just a wrench.