They are 2 different dia. so the plate will bottom out and not mash down on the bushing. They need to rotate freely when tight.
Another option maybe would be to use some bolts that are called "stripper bolts". You can get them from places like McMaster Carr. They're really high quality hardened steel, and the polished part is ground to perfection to fit bearings and bushings. The stripper bolt on the left in this picture is what I'm thinking of. How much torque were you putting on it when it snapped off? Is it a cheap ass part, or was it maybe too much torque for that size thread?
When the shackel is tightened up on the spring or perch it needs to be free to rotate. If the shoulder wasn't there to stop it, the plate would be free to mash the bushing flange down on the edge of the spring hole or perch hole when tightened and the bushing would just get chewed up.
Another way to do this job is to use a bolt G.8.8 or better and cut a piece of snug fitting tube to act as a shoulder, you could probably use a 7/16 bolt inside a 9/16 OD tube. Not sure about mild steel plates, plates made from spring steel would be better, stainless steel is a little stronger than mild and better looking. You ain't missing anything, this ain't rocket science. Get it Hot! Hit it Hard!
If you have access to a lathe you can make a new pin to match the original. Then knock out the broken pin with a hammer or drill it out. Then assemble the shackle and weld the new pin in place. Countersink the area to be welded and chamfer the pin end so you get good penetration. I think you will find that shackle pins and plates are made of mild steel. Run a file accoss yours to see if they are "hardened" steel. I made new shackles plates and pins for my 40 Ford 25 years ago and they still work fine.
I went to the metal store, got some 3/16 scrap steel, drilled holes through all 4 pieces the same, grade 8 shouldered bolt with the shoulder just the right length. i am going to safety wire them someday. this is the same kind of design as the factory used on my car.
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