[Kenth]

Here is the screw used for 1967 GTO exc. Pontiac plant according to the assembly manual. Might be correct for 1968 also.
#9423284 5/16 X 7/8.
The Pontiac plant used the #9427135 screw.
For this screw, (not the 1970 style screw as i wrote earlier), #9789994 nut is used.