To recap:
The bolt, nut and washer shown in the 60-68 Ford Car Parts (aka MPC) and the 75 edition are incorrect (SEC37 pg12 under part number 3A730, Valve Assy, and ILL 35 pg32 - REF P-4180). These are 3/8 inch hardware, the correct size is 5/16 inch.
For 1969 and 70, the bolt is 383400-S100 and the nut is 382634-S100. They quite possibly have a red dye. They are documented in their respective Mustang Chassis Assembly manuals. No washer is used.
For 1977 and 68, the bolt is 304759-S2 and nut is 34443-S7. They are documented in their respective Mustang Chassis Assembly manuals. No washer is used.
For 1964, 65 and 66 V-8 engines only, the bolt "may" be 304759-S2 and nut "may" be 34443-S7. They are NOT documented in the 64/65 Mustang Chassis Assembly manuals. The 67/68 hardware is assumed. No washer is used.
Jim