Well, a picture is, indeed, worth a thousand words!
So, the Bisson inverted muffler DOES fit on the engine, and clears the engine mounting lugs. If this were a round-cowl airplane we wouldn't even be talkin' about all of this. The problem here is that the Bisson will require extensive excision of the surrounding wood structure (specifically, the box extension to the nose ring former). And (more importantly) it will protrude by quite a bit outside the cowl and look really horrid. Well, that IS a big problem!
I can see now what the solution is here. Gotta go with the wrap-around muffler after all:
and do the necessary surgery to the firewall to clear the muffler can. There will be a lot of material removal, but it looks to me that once done, the muffler will be near fully contained inside the fuselage OML.
Here's a pic (again) of Tailskid's Evo 26GT with the wrap-around Pitts muffler, on his P-47. You can see the rework he did to clear the muffler can. That's what we're talkin' about here. Just gotta MAKE it fit!