1966 trucks had back-up lights as standard equipment. If the stepside fenders have back-up lights it is likely to be 1966. The stepside beds were the same style from 1955-1966.
My first truck was a 1966 C-30 with the 9' box. Less than 3700 were built that year, so there are not very many around anymore. The spare was under the bed on that C-30.
For pictures of that C-30 click the bottom link on this post.
