You need to do what I just did. Look in "Ford Car Parts" (aka MPC, 75 ed, available in the library) section 30, Steering, for both Ford base numbers 3A187 and 3468. I found a cotter pin, 72035-S, referenced used for what you got.
Now, I'm going to state that this cotter pin is not referenced in the "70 Mustang Chassis Assembly Manual", but it is shown on the "1969 Mustang Chassis Assembly" drawings (AM0030, pgs 31-33). Why you got both with and without uses I'll leave to the "wizards".
Jim