What size is the thread on aftermarket emblems for 67 \ 68 Barracuda?

Yes, they use speed nuts, which cut their own threads (more or less). There are two sizes that I can think of... larger ones for the fish and the scripts, and small ones for the tiny letters on the 67 nose. Something like 1/8 inch and 1/16 inch. The hood inserts also use large ones.