Where there is a will, there's a way.
What size are the collectors?
My thinking is this ( in general, as I have never hopped up a slant, but an engine is an engine, right?)
Whatever size the collectors are, that is what diameter I would make the headpipes at least.
If you want to drop size, do it like the factory after the muffler.
But for maximum flow I would carry the diameter of the headpipes all the way out.
I mean, why put headers on it at all if you are going to choke them down from the very beginning?
