You might want to look for a 55-59 12v manual-tune Chevrolet truck radio. I think the post distance is the same, and the case looks to be the same size/shape. The 1953-54 manual tune car radio has the same post-to-post distance (and the same size case) but they have an oblong (rounded corners) glass/bezel.
If you want an upgraded AM/FM, etc, in the original case, you'd need to do some fancy converting.
