Spare tire carrier - what's wrong with this picture?

[72malibu]

Restoring my dad's 75 C15 for him.

I can't seem to find a photo showing how the spare tire carrier mounts to the frame.

I think the slotted hole on the passenger frame rale is where the long bolt with the welded-on rectangular head goes, but what's to stop that bolt from turning when I go to tighten the special nut under the bracket? The slot seems too wide to keep the bolt from spinning. Something doesn't seem right.