For the record, one of my Fastbacks had a 1/4-20 nut in place of that spring nut. It is not available in the aftermarket. However, while in a local Mustang shop looking for the correct speedometer gear, I found a busted spring nut in a stripped 65 Fastback. There was the manufacturer's name stamped on the part still ledgible, 'Pal', so I did a Google search, found the company was bought by Tinnerman, did a search on their online catalog, found a very close duplicate item, called the company and got 20 samples - for free. It helped that the sales rep had a 69 Mach 1, but the bottom line is if you are up front with many companies, you can get something you need for your car. Do a Google search on refrigeration insulation and see what you can find.
Jim