If your lower stanchions attach to the body using screws that are 5/16-24 (not 5/16-18), you could install this in place of one of them.
https://www.snydersantiqueauto.com/p...7140&cat=41734
If the screws are 5/16-18 or something else, potentially you could take this one and machine down the existing post and tap it for what you need. I realize that's a bigger job.
https://www.snydersantiqueauto.com/p...7139&cat=41734