Looks like the bolts are installed from the wrong direction. Not familiar with SSBC but normally the bolt goes in from the outside and nut on the backside.
When I took apart the drums, the bolts were definitely put in from the rear. Backed up by service manual.
As I said, the bolts aren’t threaded to the top of the bolt head, and the adapter plate is threaded, so putting them on from outside-in won’t work because the bolt would start threading into the plate, then stop once the treads ran out about 1.5” from the head of the bolt. And if it did, the bolt head would probably rub, since the bolt head and the but heads are the same size, 3/4”.
