3 lots of 2 to 1 with a 3 to 1 collector at the end is what was recommended to me by a builder down under. Check out http://www.mandrel-bends.com as well for tubing. This is also a very technical book that will help with header design. http://www.amazon.com/Scientific-Exhaust-Systems-Engineering-Performance/dp/0837603099 There are cheaper sources out there, I paid $13 shipped for my copy.