Sorry to keep beating this thread, but I'm always bugged when people get too obsessed with big headers. Headers are probably the most poorly understood element of a high performance engine. Headers aren't there to assist "breathing" in the sense of maximizing exhaust flow - otherwise, all headers would be designed with the largest, shortest primaries possible, dumping into the largest collector possible, and we'd be done with it. Headers are actually tuning devices designed to assist *intake* flow, and a too-large header will not do its job as well.
Here's an article from David Vizard which does a nice job of simply summarizing the role of headers and exhaust in a high-performance engine:
http://www.popularhotrodding.com/eng...exh/index.html
And if you're more into it, and can handle a slightly more technical discussion, download the Dynomation user's manual and read the chapter on Wave Dynamics analysis from this location (warning - 40+ MB):
http://proracingsim.com/downloads/Pr...sersManual.pdf
And if you're really into it, like me, buy the actual Dynomation product and fascinate yourself experimenting with different intake, cam, and header designs and watching the effects on such things as P-V traces and HP/Torque curves.