I seem to remember them being speed-nuts, but my last '68 Decor Car restoration was 15 years ago. If you look at the threads on the studs, coarse cuts on pointed studs mean speed-nuts. Fine machine threads will of course mean captive-washer machine nuts. I seem also to recall the nuts having some rubber or dum-dum to seal them and prevent rust from carving through the barely-painted inner surface. Going even further out on this weak limb, I think they took a 3/8" socket.