I run 2 1/4 exhaust pipes off the headers into Ultra Flows and out the rear in my 69 Dart with a 416 with about 600hp. I had Flowmasters on before and I like the Ultras better for sound and flow. I unhooked the pipes from the headers at the track and gained .22, so! My suggestion would be to put at least a 3" system like gregsdart said with whatever muffler you choose and I think you will be happy. Me personally I dont have any insight on an X pipe but I have an H pipe on my Challenger and find it to be a pain in the rear whenever I have to work on anything under the car so any gain it might be making is not worth the pain IMO!!