They changed the way the oil comes up to the rockers in 58. The slotted bolt was never found on a "factory" engine, it was used by the dealers and machine shops to adopt earlyer heads to the newer blocks with the changed oil flow system ( no oil line under the side cover).
I would bolt 2 of Langdons Holley /Weber carbs on it (arround 60$ each) and you will see a boost in milage and performance.
Wow I wish something like this would happened to me
Frank
