HEI in Mopar ECU casing

I want to say that I used 6-32's to mount the module.

Not a fan of modifying 'wear' parts like the module. Parts like that need to be easy to replace in the field.

I would use flat head screws to bond the two pieces together. Call it 4-8 of them. Clearance drill & countersink one piece and drill and tap the other. On assembly coat one mating face with the thermal compound and then screw them together. Then drill & tap for the module's screws.