Ok..... looks very nice! Id buy one of these.
And I commend Horizon for showing the 2-c engine installation as a baseline!!! Big hi-5 there!
(most manuals/pictures for other aircraft chicken-out and show a simple 4-c install)
But 6.5 lbs, 600 sq inches... talking 90LX power here
Likely the Toni muffler kit will work nicely. But it would be useful to see the firewall area first.