HEI in Mopar ECU casing

Del has a good point about the top of case and aluminum.

An aluminum heatsink is about 4x better in thermal conductivity than steel.
http://www.engineeringtoolbox.com/thermal-conductivity-d_429.html
A heatsink relies on surface area exposed to air. That is why heatsinks have fins. The thickness of a heatsink is designed to carry heat away along the heatsink, otherwise a temperature gradiant across the heatsink makes it less effective.

Another important consideration is the module ground connection. It might best be be done with a ring terminal jumper connected directly on a module mounting terminal, to avoid a dissimilar metal connection if aluminum is used.

I would guess the heatsink requirement to be 10 to 30W. The module maximum temperature is likely less than 125C. I can do an estimate of size, but much depends on abient temperate and airflow.