I don't believe the threaded shackle pins and bushings are a common size or thread profile.
The good news is the bushings press out of the brackets in the frame (and spring eyes) and new bushings with pins come in the replacement shackle kits.
Picture and part numbers here and following pages
http://chevy.oldcarmanualproject.com/parts/1929_54/432.htmNew parts can usually be had at NAPA or other auto parts stores, spring shops, or the usual restoration parts suppliers.