Broken rocker arms two weekends in a row

I had the same problem with Indy rockers with 600# pressure over the nose springs, lasted about 2 years before they fatigued to the point of snapping in the same area as yours one right after another. Have two friends with big block Chryslers with Harland sharp rockers, back in the 90's with high rate springs do the same thing. They switched to solid body rockers(non roller bearing) One went to Mopar Perf. and the other went to Cranes. Like PRH said they were just to thin with the bearings in them.
Back then I ran W2's with 1.6 Mopar Perf. rockers, .630 lift and 420# over the nose for around 10 years with no failures. Sold the whole W2 setup and bought the Indy Brock heads and rockers. I've been running Hughes rockers since 2010 and on my 3rd set of 600# springs with no signs of the rockers stretching or ovaling around the shafts yet.