Those spring mount brackets on the rear axle housing are correct - they allow pivoting when the rear springs flex.

https://www.chevsofthe40s.com/detail/8661/Chevrolet_Bushing_Bolt_With_Nut_Rear_End_To_Spring.html

"These are bushings with bolts, nut. rear end pivot at leaf spring on a 1/2 ton pickup."

The bolt that goes through the bushing is often hard to remove.
Also, you might have to press out the old bushing from the cast housing, and then press/drive in the new bushing.