The FIA headers originally were of a very short primary tube length about 12", esentially a short tube into a downturned 3" collector. When they were originally designed and homologated, the header science was in its infancy (1963) and the gains available with longer primary tubes may not have been fully realized. This is what I fabricated for my application. These are for a 289.
Rick
PS:
www.CobraRestorers.com has pictures and product available of the original FIA Header.