I had the same situation with my ST 2300 and a Bisson Pitts. I could not find an extension. I used a piece of 1/4" aluminum and made my own extension (spacer) using a drill press and Dremel tool. I re-tapped the muffler for 6/32 (I think that is the right size) bolts and bought the proper length Cap head bolt from my local hardware store. I got some thin high temp gasket material from the auto parts store and cut two new gaskets.