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Be sure to look at the link that Stuart gives above. In that post,
Pre '68 Dave explains how/when that special bolt is used. That bolt does not seem to be mentioned in any of Truck Shop Manual (but might be in a Service Bulletin).
The 1960 Truck Shop Manual notes other significant changes in blocks/heads/lifters/rocker-arms between 1958-1959 that are related to the need for the special head bolt:
1958: New style mechanical lifters were used in blocks from 1958-1962. Those blocks had an oil passage in the block from the camshaft bearings to the rocker arm assembly (I think earlier blocks had a thin tube inside the side cover for this purpose). I believe this oil system also allowed for the use of hydraulic lifters in these blocks. The new lifters had an indentation in the side (an annulus) that allowed oil to pass sideways towards the middle of the block and then up.
1959-1962(63): a new style rocker arm assembly was used to allow for better full-pressure oiling. There was a new oil connector and all rocker arms had a smaller oil groove than in earlier years.
It would seem to be a good idea to keep matching rocker arms assemblies with same style heads (59-62); and, it is imperative to use the correct style lifters in the 58-62 blocks.
Here is a page with a picture of the 58-62 block . This information comes from the Chevy site of the Old Car Manual Project (thank you, Keith Hardy).
