Hi Pete,
I found an article that may help you. When you make your expansion chamber, the header pipe lenght should be adjustable. The stinger (tail pipe should also be on the large side so that you can insert a smaller tube to vary size, lenght and position. You have to play with these adjustments until you get what you want. Remember the gain comes from allowing an advanced exhust port and improved scavaging. This is true even with stock exhust timing and correct pipe configuration.

Enjoy, Dave

http://www.motorcycle.com/mo/mcnuts/em-pipes.html