Pat, agreed that mounting the connectors oriented downward might help...but then, the harness connector housing becomes a "pocket" to hold water....although it is a tad more difficult to get water in there. I would mount the ECM the way you suggested...cant hurt. I'm not sure if the factory uses some kind of grease on the connector to help the seal and corrosion problem....there are dilectric greases made for this purpose that might be reasonable to consider.

Also, I forgot to mention grounding.....if the factory grounded the case separately from the harness, do so.....for many of the ECM designs, the circuit board ground is case ground. Following the factory mounting methods can help assure good performance.