Engines are built this way---when the carburetor mounting flange on top of the intake manifold is level, the rear of the engine tilts down about 3 degrees---this is to minimize the "hump" required in the floor to clear the driveshaft. The engine is mounted to the frame, so the engine never really moves in relationship to the frame. The rearend is mounted on springs, and as the springs compress and uncompress to match the road conditions, the rearend must move up and down in relationship to the frame. Since the engine (and the transmission output shaft)both tip down 3 degrees, it is a good plan to keep the input shaft on the rearend pinion on the same angle. WHEN THE CAR IS AT REST UNDER FULL NORMAL LOAD. That way, the driveshaft universals have the "midpoint" of their flex pattern. The angle can change to a plus or minus as the rearend moves up and down, but when the rearend stabilizes (as in the car is back on a level road) then the universal returns to its "midpoint" or neutral position.----also, keep in mind that there is a universal joint at both ends of the driveshaft.-------Does that in any way answer your question???