Here are the different lengths:
FX/FJ - 10.305"
FE/FC & FB to B65280 - 10.4 "
FB after B65280 and EK/EJ - 10.212"
My suspicion is that it is actually the tappets that are from an earlier engine (with the longer rods). So you don't have rods from an FE/FC, you need them because you have FE/FC tappets.