First I would pull the center passenger side head bolt and see if oil comes out of there.
A stock 1958 235 should not need a drilled head bolt. With 1959-62 rockers oiling could be a problem.
I would suspect the 1/16" orifice used to restrict the amount of oil the rockers get is plugged.
Under the side cover at the center there is a 1/8" pipe plug. Under the plug you will find the restrictor.
If this restrictor is plugged, and you do not want to pull the side cover to check, a drilled head bolt will work in this case to redirect the oil flow.
Be sure to read my blog on this first 
