I've built lots and lots of strange motor/car body combinations over the years, and I can tell you without a doubt that having a level motor is a great goal, but not always achievable. I can also tell you that the level can be off quite a bit and not cause any issues. Some vehicles are actually built with the motors set at odd angles on purpose, they were designed so the motor was pretty level when the vehicle was operating at very strange angles.

Fuel injection makes the level motor a lot less important then the old carbs required, but even the old carbs were not as level dependent as some would want you to believe. Gene