I’m putting the rear valance back onto the Cougar now but I did not remove it originally. I have the 11 (or 9) sheet metal screws and the two bolts with the odd square tabs on one end but how do these corner bolts attach? Does the square end go through the tab on the valance and then get tightened from inside the trunk compartment? Or what?
There should be a small metal bracket that it spot welded on each end of the valance panel. The long bolt with the square tab on it will lock into a hole in that bracket. Then the stud will extend into the trunk area where you install a nut on it and tighten it. Sometimes those small metal brackets will get broken off which will have one scratching their head on how it works.
I formed a circle of Ford body mastic to go around the bolt, once positioned in the rear valence slot, to hold it in place ( like the electrical tape trick in the CJ Pony Parts video ). I also made another circle of the mastic to go around the inside of the nut fastener ( the one that goes way down deep into the 1/4 panel drop-off ). The use of these two circles of body mastic helped to keep the bolt in place as it was being tightened and also to provide a seal to the hole in the lower quarter panel, from both the outside as well as the inside.
Hope this is of additional help to you.
As an extra detail/help believe you will find that the top row of attaching screws were installed before the car and valance was painted if you looking for the original look and details