Most of the bottom flat surface area on a IO-390A sump appears to be the intake plenum. Where are people attaching their pad heaters to maximize contact with the oil sump?
I used the Reiff turbo system and put the two sump heating pads on the sump protrusions on each side. The left one was on the lateral surface as seen in the photo under the "red cube". The right one was placed on the rear surface of the protrusion because the data plate for the engine was on the lateral surface. Hope this helps.
We have the new YIO390-EXP239 (EXP119 as called by Van’s) installed and have no straight surface on the oil pan.
The only possibility I see is below the oil pan. But even here we have an imprint on the right hand side, so we don‘t have a flat surface…