Along those lines, an idea that I've never seen fully tested is a water core inside the oil pan.   The advantages would be that you don't need extra space for an oil cooler, and you have only one pressurized oil hose.   The oil pickup tube is a bit of an obstacle, but with your welding talents, I'm sure you could work around this. 

Submerging cooling coils, or a tube-fin matrix in the oil pan is not effective because the oil has poor conductivity, and is not moving.  It would seem possible that a configuration in which the oil returning to the sump (or pump inlet) had to run through the matrix cooled by the coolant could work; but the question is getting sufficient surface area for what would still be a poor heat transfer coefficient.  Plumbed in downstream from the radiator on the coolant side to increase the temperature difference can help.

Al