Sorry about 13/16 instead of 15/16. But your picture of the manual shows 273,318,340,383 are all the same., except hemi which came with Super stock springs. Of all the years stripping these cars I have never seen the 5/8 bolt on 67-72 a-body. Maybe they removed them before I got the cars. I have a couple complete cars here that are original 273 commando and 340 cars with the springs and rears in them in. You have my curiosity I will go check today. I also have the rear hangers to install early SS springs . They take larger B-body bushings and larger 3/8 shackles.
I can help you with your part but I would need you to post a picture of yours that you need a pair to. Include a scale in picture for location of hole. I get $25 plus ride for one that is cleaned to bare metal and free of rust pits. Steve