The rear fenders changed in 53 to the modern style like on the Little Red Express and remained unchanged through the end of the step side box production in the mid 80s. The rear fenders bolted to the side of the beds, so moving the holes and filling in the unused holes were all that was required to use the fenders of your choice.

Step side bed sides were a bit more complex. Dodge made low side beds, high side beds, 6 1/2' beds, 7 1/2' beds and 9' beds. In 57 (maybe 56?) the top of the bed rails went from a 45 degree angle to a 90 degree angle. I could get the book out and look up the specs as to what things were available when, if I wasn't so lazy. Most bed floors were wood planks bolted to steel crossmembers with metal strips, and the bed sides bolted to the wood floors. The front of the box contained the front crossmember and was bolted to the box sides, the rear crossmember was riveted to the lower part of the box sides (if I remember correctly). Somewhere in the 60s a steel floor must have become an option, as I've seen several "newer" beds with factory ribbed steel floors. The steel floor beds were all welded together like the wide boxes were. (Bet you can't tell I've had a few of these trucks can you? )
Gene