The two piece design is adjustable. I can build a one piece design for my own engine but I wouldn't sell it as a product since I'd get an endless stream of complaints. People build engines with blocks that have been decked or with different head gaskets or with heads that have been milled but then blame me when the valley tray doesn't fit.

The two piece design has slots so it can slide back and forth to adjust for different deck heights, different head gasket thicknesses and different cylinder head heights.