Could the dimples in your 340 oil pan be for clearance of your center drop link? It connects between the pitman arm and the idler arm. Sometimes those drop links travel pretty close to the oil pan when turned all the way in either direction. If you turn the wheel all the way left or right, you'll see what I'm talking about. Look at how the center link travels. The big block A-body conversions use the 699 or 187 oil pans that have clearance stamped into the pans.