I had a similar problem attaching the side hood emblems on the 54 and found a solution at the neighborhood Ace store. This store has an extensive hardware/fastener section. What I found was what I call a clip nut but some call a spring nut. It is rectangular black oxide spring metal with a hole that has two sprags punched on opposite sides of the hole. This fits over the emblem mounting shaft (which does not have any threads) and is pressed against the inside surface of the fender/hood/whatever so the spring forces hold the part in place and the sprags cut into the mounting shaft. If you cannot source it locally I may be able to find what you need at my store.
