Among the many differences between the Gen-3 L6 (Chevrolet, Buick, Oldsmobile, and older & newer Pontiac) and the OHC engine is that the "normal" block is skirtless (like a Gen-2): the oil pan rails and upper main bearing saddles & main cap seating surfaces are all flush with the crankshaft centerline. The OHC block is skirted (like a nailhead Buick): the block extends down below the crank, and the main caps seat up inside the block rather than on it.
Without conjecture as to other factors, the skirted block has the following advantages:
1. greater stiffness (resistance to distortion under power)
2. greater crankcase air volume below the pistons and above the oil level (reduced pumping loss)
3. greater distance from the lowest point of the rotating crankshaft to the pan rail (less windage)
