Post an ad in the Parts Wanted forum in the Swap Meet.
There is no need to indicate "stepside" because there was only one type of pickup side in 1953.
State that you want a 1953 3604 side mounted spare wheel carrier. That style was only available in 1953. In 1954, the style of the bed changed, so that side mounted carrier is a little different.
Maybe someone will post a photo of the 3604 with the side mounted wheel carrier (and a picture of the bracket/basket/mount). Unlike on the 3104, there was no need to have a dimple in the fender on a 3600.