On the Plymouth cars 71 is different, 72 is different from the 71 rear valence. The 73 and 74 will interchange with each other.
For 73/74, Even the gas tank on a 74 is different than the 73.
We were talking about differences between 71/72 and 73/74 so I added my 2 cents how even the 73/74 cars were different in some ways.I don't think that affects the valance.