The parts car is gone, there is a couple salvage yards in GA that specialize in fox era mustangs, google will help. Post up if you find some cause I have a couple more places here to look.
I made similar brackets for my aftermarket Cobra rear bumper (I didn't want to use double sided tape). Don't see why they would not work for your application.
I used 1/8" flat bar (I think 1" wide, but 3/4" should work fine) and welded some 1/4-20 carriage bolts on the side of the flat bar.
You could use some heavier gauge sheet metal to replicate the shape of the other bracket, drill holes where needed and use hex bolts of your choice (probably best to tack weld the bolt heads). Again could use 1/4-20s or just smaller.