if u do not want to cut clearance for mufler and dont want to buy pitts mufler, u can also buy the muffler extention fron os, its about 10$, it fitts 61fx and 91fx as well... works perfect, but might have to change carb settings...

re-drilling holes on firewall, dont think is dificult, i even used the same blind nutts...

ps: in this pic, i rebuilt the landing gear mount and therefore i changed the clearence i made for the old mufler too, since it was not going to fit with the new 91 fx..