Check my sig for specs.....
Right now I'm running a dump on the single section of the y-pipe right before the cat. It has definetely helped out, though I think the single 2 1/2" pipe and the lack of scaveging is hurting me.
So I'm basically looking to do this:
Run dual 2 1/2"s from the headers back to the rear axel but not over. Route both pipes on the passanger side as well as two gutted cats (for visual just incase). I would also like to add an H-pipe in there somewhere as well, though it would probably only be a few inches long, as the pipes will be pretty close to one another. No mufflers.
Does the pipe size sound alright? I'm thinkin' dual 3" may be a little much. Also, I would assume the H-pipe would be benificial, but would it be worth it to try an X pipe? I'm basically looking for something with very little back pressure and a good scaveging effect.

 

Any scavaging effect is pretty much over at your header collector. So, all you're looking to do from that point on is reduce back pressure.

An X- or H-pipe isn't going to have significant effect in your system. You could look into a dual-in/dual-out cat, but I'm not sure if 2-1/2" inlet/outlet versions are available.

I'm running 2-1/2" tailpipes (non-mandrel bent) on the '57. Before I had them installed, I was dumping the mufflers under the car. I had a lot of 1/4 mile runs to compile data, and when the tailpipes were added, they didn't affect ET or MPH at all. So, 2-1/2" is adequate for my application, at least. Should be fine for yours, too.