Until the repo pedestals were released, you couldn't get anything for a '71-3 other than the complete spoiler. From Ford, the pedestals and studs are an assembly and came only on the spoiler. They were never released separately as a service part, so there was never a part number assigned to any of them by Ford.
The nuts that thread onto the studs from the underside of the deck lid are ¼-20. Unfortunately, neither of my Money Pits have rear spoilers, so I can't get a length measurement. As I had suggested to another forum member with the same problem, you could always use something like a small dowel rod and insert it into the pedestal while placed on the deck lid. Once you determine how far into the underside of the deck lid the stud needs to protrude, you can take some of your hard-earned money to your local hardware store. There's a good chance they may already have some pre-cut studs the size you need. If not, you should be able to find some threaded ¼-20 stud stock and cut it to size.
Good luck with your project.
