The following statement is from Inland Empire's Power Train Setup documentation posted on their web site.
"Engines are set in the frames at 3 degrees downward angle to the rear. This is virtually a world standard for engine placement and is the basis for intake manifold construction. It is this fixed installation that determines the possible. To arrive at equal and opposite u-joint angles the pinion must be set parallel to the crankshaft."
Actually you might want to read the entire document as it also talks about engine placement in the frame.
http://www.iedls.com/asp/admin/getFi...&TID=28&FN=PDF