HEI in Mopar ECU casing

Notice the white oozing out around the edge of the module. This is the heatsink compound. It helps to transfer the heat away from the module. You can bolt the module to a piece of aluminum w fins cut into it, or in my case its a 1/4" thick aluminum plate. Aluminum will draw the heat away better than steel.

Make a 1/4" thick square aluminum plate for inside inside the box that attaches to the now "fake diode" thats on the outside, drill, and tap the aluminum.block, and mount the HEI module inside the box on the aluminum plate with heatsink grease. Thats solves your heatsink, mounting, grounding issues in one shot.