your race header will eliminate the cat completely...you do not NEED a flex pipe...the header will bolt directly into your stock flex...though i believe it is much better to get the larger diameter flex...just to be on the safe side...remember...your exhaust is only as good as the smallest part
what about a shorty header? I've heard that those are better in the lower rpm/speed range (which is all I need for everyday driving.) Which parts do you need to have, and which are eliminated? Also, I think Maine has adopted Califonia's emissions standards, so I'd need to be legal.