The horns both have two prongs for aligning and a single bolt hole. They come as a left and right assembly. And they simply mount onto the face of the plate that is part of the core support bracket that has 6 holes in it.
79-81 Firebird front support bracket horns nosecone 4 by
Aus78Formula, on Flickr
79-81 Firebird front support bracket horns nosecone 3 by
Aus78Formula, on Flickr
The horns actually have a nut that secures them to their mounting brackets but normally don't come apart. That possibly explains why one side also turned slightly as nipped up tight by hand.