I'm currently building the Sig Ryan. My engine will be an inverted OS 61 hellicopter engine. I decided to go with a Dubro aluminum mount with the rubber bushings. It's quite a bit longer than the supplied mounts. I located the drive flange per plan and slid the firewall back the appropriate amount. No problem as far as I can see. I did trim the fuselage sides a bit after sliding the firewall back. My problem now is locating a good, inexpensive Pitts style muffler for inverted position.