If you it’s a 58 you are working on, you want the 55-58 center groove rocker set. New, these are really really hard to come by. GM changed the oiling system in 59. This was the last update to those motors so the 59-63 rockers are different, including the offset groove. These rockers are a bit more easy to find new a trw and sealed power sold new sets for far longer then they sold new sets for the 55-58 version.
If you have a set but they are pretty used up, you can have them rebuilt. The only place that I am aware of that can do an excellent job is Rocker Arms Unlimited in California.
http://rockerarms.comThey rechrome and remachine the shafts, machine the arms, adjusters, and you have a nice set ready to go. Not cheap I know but you would be surprised at what a good set of rockers will do in terms of a good running motor. Worth it in my opinion. If you go this route, and while the rockers are out getting rebuilt, now would be the right time to pull the side cover and remove the feed line, clean it and the fittings out, and make sure the passage through the head is clean and clear.
Once it’s all back together and running, you must must must make sure the rocker arms are dribbling enough oil to lubricate the tip and adjuster of the rocker arms. If the shaft is not getting enough oil to feed the rockers, you will make fast work junking that nice new set of rockers.
If you discover that you are not getting sufficient oil, then plan B which is the external line feed.
