http://www.fordpowertrain.com/Headers.htm
Don't know if Jay's dyno test can be searched here anymore (as some E-Pay sellers were stealing his results and using same to sell their parts). His upcoming book would carry the extensive dyno test results on logs vs. Ford's cast long tubes vs. aftermarket steel tube, etc.
That said, IIRC the logs easily lost 30hp or so when compare to true steel tube headers. This is the real log-jam (hah, sorry!) in any FE build so adding the FPA headers not only frees up that HP but makes the downstream mods even more effective. Unless funding is tight, headers, a 2.5" X and 2.5"s all the way through is the way to go IMHO.
Muffler selection is important too as why add a nice system with restrictive mufflers! I'd use the search function and check on many recent muffler posts regarding sound, flow, etc.
