The motor angle is dictated by the pinion angle of the diff.
If the pinion is horizontal the engine should be within a degree or two of horizontal.
If the engine is in a different plane you can get driveline vibrations due to the universal joints working against one and other.
When a uni joint rotates with a bend in it, it slows down and speeds up twice during a full revolution.
If the two joints are at different angles to one and other it stresses the driveline and causes vibrations.
Conversly a driveline shouldn't be run in a straight line as the uni joints can suffer premature bearing failure as the needle bearings don't rotate correctly.