I see that Miker mentions Hedmans Tri Y firing order specific headers. I am puzzled as to the construction of such a device and looked up pictures of these items.

How do these tri y  qualify as "firing order specific".
A true F.O.S. design incorporates the tubes from each side to scavenge the gasses as each cylinder fires in its order taking advantage of pulse theory.

F.O.S. headers usually have a pretty curious appearance and generally wont fit into a conventional automotive frame.
So how do these headers qualify under a firing order specific label?
Oldmics