RE: H9 Sukhoi Motorbox
Sorry I am not mike , but I would try to make the opening in the firewall as small as possible. I have this plane with a DA 100 and I love it! How far out of the cowl will the prop hub be if you just use stand offs? This can also help you out as for cg issues. My plane is 25lbs with the DA and the cg is right on the money with no added balast. I also have a pull pull setup for the rudder.
Fred