Here's the way I did mine...
I used a stiff wire and taped the nut/plate to it pushing it up from the bottom. After you get the bolt started pull the wire out. If you remove the shoulder bolt at a later date you just need to repeat the install process again...no rivets were needed.
I drilled my hole 36" above the floor.
I used Juliano's Seat Belts for both of my '56 trucks and had them installed in less than 1-hour (small window cabs). You may need to trim the nut/plate to fit inside the smaller door post, just keep it a big as you can...surface area counts here!
Mike B
