The beds and tail gates are the same from 54 through about 1980. I am not sure of the year of the step side change, but there are many years to choose from. If you are looking for complete originality the 54 and 55.1 beds are different from all others. They have a raised area moulded into the bed side that follows the fender contour. The 55.2 and later beds have smooth bed sides.

54 Chevy build pics.