Bertoia molded shell side chair hardware - help?  

I’ve been restoring some Bertoia molded shell (fiberglass) side chairs from 1970. I had to detach the shells from the metal bases in order to strip paint that someone else applied in a previous restoration attempt. Now I’m having a really hard time reattaching the bases. When I try to screw the bolts into the specialized nuts/tabs, it’s nearly impossible to get the parts to connect. The tab keeps slipping out of the way; I can’t get the bolt to catch, etc. Does anyone have tips for how to work with the hardware? 

Thanks in advance!