1989 black lightning down tube shifters

I have a challenging issue. After stripping and repainting the stock frame i have attempted to put the Dow tube shifters back together. The double threaded stud that connects both bosses to the frame got damaged somehow and needs to be replaced. I have learned that is a M5-.8 x 55mm double threaded stud screw and I’m looking to get a slightly longer one, maybe 70mm. I don’t know where to fine this part and it is the only part that i need to finish my project. Please assist.
 
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