The CMB club started a few years ago a Sport D (40) tunnel class to be run at the Tunnel Champs race. It is not an approved "stock" class as one does not exist at present. So we created it by using any 40 engine, bolted onto a OB lower, and must have a can muffler - no tuned pipes. All else is fair game. That is how you can unbolt a can muffler after running in the Sport class, hang a tuned pipe on it and run it in the modified class.
I believe the class is called Stock in D3 Storm series, but we call it Sport D tunnel at the Tunnel Champs. It is essentially the same. The intent was to try and get the attention of some manufacturers, besides K&B, to build a complete engine. It has not happened yet. We did not want to structure the class to only one engine builder so we came up with the current rule.