Hey Lowns,
Engines are designed to sit at an angle of about 3-4 degrees sloping down from front to back. Intake manifolds are machined at that angle the other way so the carbs sit level. On my engine the cover brackets are made at 4 degrees the other way so that the engine looks level in the car. I think the main issue is to get the drive line angles correct. Typically the differential is also mounted with nose up 3-4 degrees (although my truck was set at 6 degrees). When the drive shaft is installed then the "working angle" of the universal joints (that is the angle between the transmission tailshaft and the driveshaft, and the angle between the differential output and the driveshaft) is equal to within 1/2 degree, greater than 1 degree and less than 3 degrees. That way the universal joint forces, front and rear cancel each other and are not large in any case. So why not run a straight shot between the engine and differential? Then the universals don't "work" (the needles in the universals don't roll) and the needle bearings "brinell" on the race and wear out.
Here is a good link on all that. More than you wanted to know probably. I don't want drive line vibrations messing up my ride.
http://www2.dana.com/pdf/J3311-1-HVTSS.PDF
TR