Just a suggestion for you. A full rebuild in my book would entail new rocker arm assemblies as well.
BTW, if the rocker arm slot was not long enough, more than just one would have broken. Some
day, someone is going to develop a see through valve cover for engine break-in so we can watch for
things like this.
I'm toying with opening up a top window in a set of big block valve covers and placing in a cap so I
can remove it from time to time during the run-in phase every time I build a big block.........