The answer to your question is shown in the 1969 Mustang Body Assembly Manual, AM0026, pg 8 (ref N9-105-2). The washers are used as shims, along with some caulk. The assembly manual also shows that the upper A-Arm shims, C4ZA-3043, (thickness not specified may be used (no verification of this use).
I suggest you get a set of the manuals.
The bad news is that there are three pages missing for the 69 manual, ref N9-105-1, N9-105-3 and N9-105-5. You may use the 1970 Mustang Body Assembly Manual pages N0-105-1, N0-105-3 and N0-105-5) as reference as they are "close", but not exact (the headlight difference).
Also, in Ford Car Parts (aka MPC), located in this forum's library, there may be enough information to help you out.
Jim