I have the same engine in a Hangar-9 P-51 150 as well, can you tell us which muffler you have now? There are a couple of different ones out there (mostly bissonmufflers.com) or if you have an unlimited budget you can go to keleo-creations.com and he'll make you anything you want. (I have one of his for my Saito 182 twin that is very good).

MadDog Joe