There is an old Mopar yard that might be able to help. Call Gary at Winkle Auto. 603-673-4616. He drives cross country and buys and parts out old Mopars. He may be able to help.
Late 1970's cars as a group were bad although it seemed that the Mopars then were the worst of the Detroit 3...............It's a shame but those 78 cop cars just rotted away bad. I had one that came from the west coast and was rock solid. Most of the ones I looked at locally (this was in Ohio) were rotten in so many places. This want Chrysler's best time for corrosion resistance. Hope you can find the parts, they are awesome cars.
The R body replacements were much better at resisting rust. Not perfect, but way better.