I would get the FC 4-2-1 header, it flows better when it goes from 4 into 2 into 1 and I believe it is a slightly larger inside diameter ( not possitive though). They do use it on their 12 sec all motor car so it has to be pretty good, I love mine and would buy it again if I had to. Just make sure you get a larger flexpipe and exhaust or the header really won't help that much.
2001 sangria zx3, best 1/4 time 15.70 @ 85mph. Now have an 04' Pontiac GTO w/LS1