Since the thrust line is not necessarily at the centerline of the firewall itself (since we don't know which plane this is ), I don't think you will be causing any aerodynamic problems by having the centerline of the engine one inch below the centerline of the firewall. However, if the plane was designed such that the the two are supposed to be in the same place, then I think you may have a very minor trim issue: When you increase throttle, the aircraft may tend to pitch up slightly. Key words being very minor and slightly, if anything. If that is noticeable to you, then just increase the downthrust a little. More than likely, nothing will seem amiss. OTOH, 1" isn't all that much on a 1/3 scale plane, while 1" is a pretty significant number on a 36" wingspan plane.