The spring center bolt isn't part of the spring seat, but is probably rusted into it. A spring shop should be able to get you a center bolt. They have a round head instead of a hex head (or you could machine off a hex head bolt.) But if you're replacing springs, they will come with a center bolt.
The nut and bolt should be available at a well stocked hardware store, or online at McMaster-Carr. You have the size from the existing bolt and the bushing is "glued" in place by the rubber in the spring seat.
If you're looking at the figures in the Factory Assembly Manual, they're for years after GM quit using the pivoting spring seat.
