ORIGINAL: Stew99

The spinning propellor is putting an opposite torque on the fuselage making it want to roll to the left. It also sets up a spiral slip stream around the fuselage that hits the vertical fin on the left side making the plane want to yaw to the left. These effects vary from one design to another so it's usually best to start with whatever the designer has specified. After you see how the plane flies in a stock set-up you can begin to experiment with other set-ups. As someone else mentioned, after mounting, the engine thrust line can be shifted with shims or washers under the engine mounting lugs or between the engine mount and the firewall.