I have the muffler of my Saito 180 tucked in the trough like in post 378, except without the rubber exhaust extension and that has worked fine for me. I also had to grind a hole in the firewall so the pressure nipple wouldn't rub. I haven't seen anybody do what you are proposing. If you go ahead with that, I'd probably reinforce the cut-out in the fiberglass with some plywood backing to prevent flexing and cracking around the muffler.