OK I just went and took a look at your configuration looks nice. The nipple configuration I came up with allows me the right amount of distance to place the muffler spout right out through the very bottom opening already present in the cawl, No cawl cutting what so ever with no rubbing on any part of the cawl/plane. Since this configuration is quite shorter then what you used, I believe it will help on the vibration issue. I just wish I had saw your post and responded to it before you cut the cawl.

If I get the chance I will take some pictures but I think you have a good idea of what I have.

Randy