Mine was also missing on my 73 Cuda, I did a similar thing to the cage nuts Wild RT has suggested but as I didnt want to open the hole up any further, I just mad a square nut with a flange around the base so it wouldnt pull through, a little dab of silicon to hold it in place and done, if you have access to heat you could get a regular nut to suit, square it up to fit and solder a round washer on the back side of it to stop it pulling through etc.