HEI in Mopar ECU casing

I would grind off the lip of the ECU and just use the one peice of aluminum with taper head machine screws from the underside up to mount the ECU with nuts on those screws.
This way you only need one peice and it would also give a good ground when you screw it to the body metal.
Then tuck your wires into the underside of the Mopar box and mount it over the ECU.