A better option is to use the Great Planes engine mount:
http://www3.towerhobbies.com/cgi-bin...?&I=LXJ772&P=7
Its width is adjustable, so if you ever swap engines, AND if the new engine is wider, you can buy an identical mount (if you need to re-drill the holes) mount it to the same blind nuts that are in the firewall, and spread it apart a little