Thanks everyone!

I figured it out...

From 1963 to 1966, the bolts were 7/16 – 14.

From 1967 to 1976, the bolts were 1/2 – 13 (as others have noted).

Hence, this means that the K-Members (with respect to the Steering Gear Box only) are different between 1966 and 1967.

And to make matters worse, the Steering Gear Box bolt holes are different also. (Well, at least one of the holes for 1966 is different... the two that are together on the same side will fit both sizes).

Meanwhile, I was able to locate my missing bolt...