I'm using Kirkham headers and also cut out their mufflers and put in Howe racing mufflers. 3-1/2" core, 4" outside diameter. I also put them on backwards so the louvers face the back and they sound awesome on my Keith Craft 496 FE.
Several others in the area have also used the Howe racing mufflers...
Here is what they sound like....This was in June in Dallas about 98 degrees out and had some crappy gas with entanol in the car and the gas was boiling in the carb so was hard start that day........now mix some 110 with it....
Shelby Meet!!! - YouTube