I do believe you looking for the bumper fillers on the Ft of the car or if it is in the rear the were a solid part of the rear filler panel.
Wish I was better with computes as I could post some links as to the different Tail panels and steel structures for the rear but I can tell you that !
What ever rear filler panel you choose it will bolt in to the sunbird . With that being said you will have to modify the steel tail light mounting to accept the tail lights . Here is a , pic that I 've posted before for other members .
Last%2520Roll%2520-%25203-1.jpg
This is a 80 sunbird with a 80 monza flat panel tail light rear facia attached with the sunbird tail lights.
It is the only found quickly .
Perhaps another member has the other picks of the tail light mounting differences that I have posted handy and could provide the links
Len