Depending on your engine setup go with 3.5" in to 3" out x-pipe. The x pipe will help a lot with eliminating drone. No experience with the super scavengers but the long collectors might make it hard to fit the 3.5" inlet x-pipe.
If you are going to a free flow system like this do some homework on muffler selection so they don't end up being the bottleneck in the system. Mike D.
