You guys may enjoy inputting your engine specs and seeing what the calculated header dimensions are for your engines at HEADERDESIGN.COM. This site is helpful for comparing how each variable change effects the ideal header design dimensions. There are other places, I am sure, but this one seems to parallel the conclusions that Chris Hill does at S.P.D. that supplies my collectors. Try changing the horsepower amounts to see how it effects the primary ID dimension and the collector ID. You have to sign in to use it, but it is free and useful info if you are trying to find the best header for your car from a mass producer. One supplier may make there one size fits all design that actually fits you best. But you have to know what you are looking for and not just purchase what a so called credible header producer has on the shelf that fits your chassis. No two engines will have the identical pipe needs so knowing what your target is will allow you to compare what is available that fits you needs best , or suffer with a nice looking set that perform poorly due to improper sizing.
Bruce