The J-hooks are not designed to go through the backing plate. They hook into a small L shaped bracket which is spotwelded to the backing plate. (That explains why they are "too short.")
If there is no evidence that the backing plates ever had these brackets, such as marks where the spotwelds used to be, then I would surmise that they do not require these springs. Some years did not have them.
Carl


1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission