It is interesting that he went to such trouble to get equal length primaries when according to David Vizard they do not have so much influence on performance in a 4-2-1 exhaust.
I agree and I believe that john Shankle designed his headers on a dyno. I made the mistake of thinking that I could gain some HP in the 6000 - 6500 region by shortening the secondaries. I did gain some there but I lost much more around 4500. On a race car that might be a decent trade but John got it right for street cars.The “what’s the best header“ question can really only be answered knowing the rest of the engine build and the car’s intended use. And even then, it often comes down to experimentation!